Ralts can be found on Routes 208, 209 and 212, only in Platinum; in Diamond and Pearl, it is only available in the postgame.
By far the easiest game in which a player can obtain a Ralts, which is ironic, considering that the Sinnoh generation comes past Ralts' own. While Psychic-types are considerably less powerful in generations from the fourth onwards, Gardevoir is still very good, courtesy of Psychic threats not being overly common in the game, and its special bulk being fantastic. The Speed is where it falls short: while a base 80 stat is not bad, there will actually be a good few Sinnoh threats that have higher Speed values than that, and this will actually prevent Gardevoir from sweeping some of what would have otherwise been easy to defeat. Even then, Gardevoir is surprisingly powerful, with a not-so-late final evolution and early access to Psychic and the awesome TMs it gets. While Gallade's typing may make it preferable in a number of situations, a specially oriented nature and/or stat asset will obviously favour Gardevoir, which can still prove its worth in the Sinnoh league.
Gym #3 - Fantina (Hearthome City, Ghost-type): No. There is no way for Kirlia to be successful in this gym, slow and weak as it is to Ghost-type moves. Avoid at all costs.
Rival (Hearthome City): Kirlia should avoid fighting Staravia, as it can 2-3HKO with Wing Attack before Kirlia gets to do the same; it will be fine against Buizel if it heals instead of switching out, since its Pursuit is only a 3HKO if Kirlia stays in and Magical Leaf 2HKOs it. Unfortunately, no dice against Prinplup, whose BubbleBeam outdamages Magical Leaf, coming close to a 2HKO. Kirlia is more fortunate against Roselia, which it can 2HKO with Confusion, but Grotle's Bite is a decisive no for the matchup against the Grass starter. Kirlia is also at odds with Ponyta and Monferno, the former due to Stomp being a near-2HKO and having a high flinching chance, the latter due to Flame Wheel being a 2HKO.
Gym #4 - Maylene (Veilstone City, Fighting-type): Gardevoir's matchup is excellent in this gym. Meditite struggles to deal much damage with Rock Tomb, so Gardevoir should exploit the first few turns to set up Calm Mind. If Gardevoir already knows Psychic, it can OHKO all of Maylene's team starting at +2 Special Attack; if its best move is Thunderbolt, instead, it will need to be at +4. There are no specific risks involved, as even Lucario's strongest attack - a five-hit Bone Rush - can only chip away about 70% of Gardevoir's health.
Rival (Pastoria City): Thunderbolt destroys Staravia and Buizel, Psychic OHKOs Roselia and Monferno, and also 2HKOs Grotle and Ponyta. None of them can 2HKO Gardevoir.
Gym #5 - Crasher Wake (Pastoria City, Water-type): Gardevoir can easily Thunderbolt his Gyarados to death, and can either OHKO Quagsire with Magical Leaf or 2HKO it with Psychic. Its matchup against Floatzel, instead, is bad: Crunch 2HKOs and Gardevoir cannot OHKO whatsoever.
Cyrus (Celestic Town): Sneasel can 3HKO Gardevoir with Ice Punch if its best move is Thunderbolt, but a Focus Blast will wreck it. Golbat is OHKOed by Psychic, and Murkrow is OHKOed by Thunderbolt; beware, however, that a critical Faint Attack from the latter will off Gardevoir from full health.
Rival (Canalave City): Thunderbolt still OHKOs Staraptor, but a critical Take Down will also off Gardevoir if it hits; the circumstance is, however, rare enough that this should not be a problem. Heracross is 2HKOed by Psychic but also 2HKOs with Night Slash, and its Speed is slightly higher than Gardevoir's; if Gardevoir is higher in level, or has a nature that boosts Speed, it should still come out on top. A healthy Gardevoir can OHKO Floatzel with Thunderbolt and also Roserade and Infernape with Psychic, whereas Rapidash is a 2HKO but still perfectly safe, as all their moves are 3HKOs at best. Instead, it should avoid Torterra, as its Bite is a near-certain 2HKO and Psychic is not sufficient to compensate that.
Gym #6 - Byron (Canalave City, Steel-type): Magneton is OHKOed by Focus Blast; it can 3HKO at best, so the first or second miss will not be a problem if it happens. Steelix is also an OHKO, but Earthquake is in the 2-3HKO range, so there will be less wiggle room for Focus Blast to miss. Bastiodon is OHKOed and should not be fought with any other move, as its Metal Burst will otherwise off Gardevoir; Stone Edge 3HKOs it, so once again, misses are not a huge issue (if Focus Blast still has any PP left).
Saturn (Lake Valor): Golbat and Toxicroak are OHKOed by Thunderbolt. Toxicroak's base Speed is slightly higher, but even if Gardevoir does not manage to outspeed, its best move is Poison Jab and it can only 2HKO. Gardevoir should not fight Bronzor: while it can win, it will needlessly wear down Gardevoir in the process, spamming Shadow Ball and potentially lowering Special Defense while doing so. Gardevoir should be kept healthy for Saturn's bigger threats.
Mars (Lake Verity): Golbat is yet another Psychic OHKO, and Purugly can be offed by a Focus Blast or two Psychics. Its Faint Attack is only a 3HKO, thus not scary. Bronzor, like Saturn's, is defeatable, but Gardevoir should preferably keep its health high for the other two Pokémon.
Gym #7 - Candice (Snowpoint City, Ice-type): Focus Blast OHKOs Piloswine, which only 2HKOs with Earthquake; if the first one misses, however, this may be a problem for Gardevoir if it does use Earthquake on the first turn. Sneasel also 2HKOs and gets wrecked by Focus Blast, providing it actually hits; Sneasel will also outspeed, so a damaged Gardevoir should be healed or switch out rather than fight it right away. Against Abomasnow, misses are more than ever a problem, as Focus Blast and Wood Hammer both 2HKO; Abomasnow may only go down in one turn if Focus Blast deals just enough damage for the Wood Hammer recoil to do the rest, and it might also just use Avalanche instead, achieving the same damage throughput. On the upside, Focus Blast will bring Abomasnow down to Psychic KO range, if Candice is already out of healing items. Surprisingly, Froslass' Shadow Ball is a 2-3HKO whereas Gardevoir 2HKOs it with its own Shadow Ball, but this is not taking into account that Froslass has Double Team and may even profit of Snow Cloak under the hail. If both of them are in effect, Froslass should preferably be taken down by a team member with a perfectly accurate move.
Cyrus (Galactic HQ): Sneasel only 3HKOs with Ice Punch and, barring consecutive misses, gets OHKOed by Focus Blast. Psychic is near-guaranteed to OHKO Crobat, though its Supersonic and Bite (flinching rate) may impair Gardevoir's sweeping; Bite is a 3HKO, at any rate, so a Gardevoir not affected by confusion should not find it troublesome to net the KO. Honchkrow nearly kills with Faint Attack and can live a Thunderbolt, so do not let Gardevoir near it.
Saturn (Galactic HQ): Like the Lake Valor confrontation: Golbat is more like Gonebat, Psychic also OHKOs Toxicroak. Bronzor may be worn down in absence of other options, but if there are any, Gardevoir's health should be saved for Saturn's other Pokémon instead.
Mars and Jupiter (Spear Pillar, tag battle with rival): Since each Bronzor can only be 2HKOed by Focus Blast, a notoriously inaccurate move, Gardevoir is better of setting up Calm Mind against them before attacking. At +6, Focus Blast can OHKO even if Light Screen (which Mars' Bronzor knows) is still up. Gardevoir can also OHKO at +2 without Light Screen. Both of the Golbat will be OHKOed by Psychic at any rate. Purugly is OHKOed by Focus Blast even at +0, though its Shadow Claw is a potential 2HKO with a high critical hit rate, and Gardevoir is unlikely to have all of its health at this point; strange but true, however, a fully healthy Gardevoir can survive even Skuntank's Night Slash, although with just a sliver of red health. If Gardevoir plays its cards well, it can do a good job in this double battle: it should first stack up as many Calm Minds as possible, then defeat all of Mars' Pokémon, the ones on the left; if this happens, and Jupiter still only has Bronzor out, taking out Purugly is possible, however risky (depending on Gardevoir's current health level). After the left side of the battlefield remains empty, Gardevoir can proceed to sweep until Skuntank comes out. If backed by a +4 or above in Special Attack as well as the certainty to outspeed - for which it will need a Speed value of 106 or higher - it can OHKO even Skuntank with Thunderbolt.
Cyrus (Distortion World): Houndoom's Dark Pulse is only a 2HKO, and Gardevoir has great OHKO chances with Focus Blast. It can also OHKO Crobat with Psychic, as its Cross Poison is only a 3HKO; do not let a confused Gardevoir finish Crobat off, however, as it will need to be healthy for the following fights. Thunderbolt OHKOs Gyarados, but Gyarados should not be fought anywhere below 80% health due to its Quick Claw and Giga Impact. Do not fight Honchkrow or Weavile, they will destroy Gardevoir before it can do anything.
Giratina (Distortion World): Giratina can annihilate Gardevoir without batting an eye. Do not use Gardevoir here.
Gym #8 - Volkner (Sunyshore City, Electric-type): Gardevoir can take up to four Iron Tails from Jolteon, and should use Volkner's lead as an opportunity to set up. At +2 Calm Minds, Gardevoir's Psychic OHKOs Jolteon, Raichu and Luxray; Raichu's Signal Beam almost 2HKOs, however, so Gardevoir will likely need to be healed before KOing Raichu. At +3, Gardevoir can also OHKO Electivire, but Electivire is faster and may deal up to about 85% with Giga Impact; Gardevoir should not fight it if its health is not full.
Rival (Pokémon League): Thunderbolt still offs Staraptor, but so can a critical U-turn from it. Gardevoir has an OHKO chance against Heracross, though Heracross will 2HKO with Night Slash if Gardevoir fails to bring it down. Floatzel's Crunch is a 2HKO, so Thunderbolt will OHKO safely unless critical hits occur. Likewise, Roserade can 3HKO at best, and gets destroyed by Psychic. Infernape 2HKOs with Shadow Claw and, although Psychic OHKOs it, this does carry a critical hit risk, which is higher than normal due to Shadow Claw being an increased critical hit ratio move. Rapidash 3HKOs with Fire Blast, and is 2HKOed by Psychic. Focus Blast 2HKOs Empoleon and, in most cases, so does its Shadow Claw; a miss will be the end of Gardevoir, so preferably resort to another Pokémon if available. Snorlax and Torterra are both bulky and have Crunch; Gardevoir cannot win against them.
Elite Four Aaron (Pokémon League, Bug-type): Since Gardevoir is likely several levels higher than Aaron's team, it should be able to outspeed everything on it. Yanmega is OHKOed by Thunderbolt and Heracross by Psychic; Vespiquen, Scizor and Drapion are all 2HKOed by Thunderbolt, though all of them minus Drapion can also 2HKO (Drapion can only 3HKO, as it has no Dark STAB). Setting up Calm Mind is not advised, as Yanmega can U-turn and only two or more Calm Minds can guarantee a slew of OHKOs.
Elite Four Bertha (Pokémon League, Ground-type): Whiscash is great setup material: its Aqua Tail is a 4HKO with a very small 3HKO chance, and two Calm Minds are enough for Gardevoir to sweep all of Bertha's team with Psychic. If Gardevoir knows Grass Knot, it only needs one Calm Mind to OHKO everything (Psychic will be needed against Gliscor).
Elite Four Flint (Pokémon League, Fire-type): Gardevoir can take up to two Dark Pulse from Houndoom and its Focus Blast OHKOs, so only repeated misses will be a problem. Flareon is a mixed bag: while Trace specimens may want to trace its Flash Fire to make the rest of the matchup against Flint much easier, its Giga Impact can 2HKO and Gardevoir can 3HKO at best. This will not be a problem if it can outspeed and is still at full health at the time Flareon is sent out, providing that Giga Impact does not result in a critical hit; Flareon's Quick Attack cannot finish Gardevoir off after just one Giga Impact, either. Any Gardevoir can take on Infernape if still at full health, or around there; Flare Blitz is a 2HKO and Gardevoir can OHKO with Psychic, though under Sunny Day Flare Blitz does have an OHKO chance, so do not try that. Gardevoir is also near-guaranteed to 2HKO Magmortar, and should also outspeed due to its level advantage; Magmortar's Flamethrower can only 2HKO even under the sun. Tracing or not tracing Flash Fire will, instead, make a difference against Rapidash, who can 2HKO faster than Gardevoir's Psychic with its Flare Blitz.
Elite Four Lucian (Pokémon League, Psychic-type): Mr. Mime's strongest attack is Thunderbolt, which Gardevoir can take very well. As such, it can set up Calm Minds in front of it. At +2, all of Lucian's team minus Bronzong is OHKOed by Shadow Ball, and Bronzong is 2HKOed; starting at +4, instead, even Bronzong can be OHKOed, with Thunderbolt. The only Pokémon on Lucian's team that could outspeed Gardevoir is Espeon and Alakazam; Espeon's Shadow Ball is only a 3HKO not counting Gardevoir's Special Defense boosts, and Alakazam has no Ghost coverage. This sweep should not be difficult.
Champion Cynthia (Pokémon League): Spiritomb has not one, but two STABs that are strong against Gardevoir, and should as such be avoided like the plague. Roserade can be OHKOed easily with Psychic, but Gardevoir should not attempt a setup against it; its Sludge Bomb is a 3HKO close to 2HKO range, and Roserade may also outspeed Gardevoir. If anything, Gardevoir should set up against either Togekiss or Milotic, both of whom can 3-4HKO with their strongest moves, on average; Togekiss is best, since its Air Slash accuracy is hindered by Hustle, but Milotic will also work. At +3, Gardevoir can OHKO Togekiss and Milotic with Thunderbolt. Gardevoir should not stop setting up before it can OHKO Milotic, otherwise its Mirror Coat will kill Gardevoir. Lucario is OHKOed by Focus Blast and can 3HKO with Shadow Ball or Stone Edge at best, though Gardevoir in low health should be wary of its ExtremeSpeed; heal before fighting it, if possible. If Gardevoir is already at two or more Calm Minds, Lucario will be OHKOed even by the much more accurate Psychic. Lastly, Garchomp gets OHKOed by Psychic only at +4 or above, but it also deals up to 85% or around there with Earthquake, so it should only be fought by a healthy Gardevoir backed by the right amount of stat boosts; otherwise, Gardevoir may (likely) be outsped and killed before it can do the same.
Post-Game: This is where Gardevoir takes a holiday. The legendaries to be fought are generally advantaged against Psychic-types, and Gardevoir is not exactly known for its physical bulk and Speed.
The initial moveset of Ralts consists of Confusion, Double Team, Teleport and Lucky Chant. All of them are useful in some ways: Confusion for obvious STAB purposes, Lucky Chant to avoid pesky critical hits (especially for setup builds), Teleport as a substitute for Fly for quicker healing, and Double Team being the most luck-reliant of all; nevertheless, still better than not knowing how to deal with enemies in situations where no other options are available. Kirlia then learns Magical Leaf at level 22, which is a very good move to have and has the plus of not being affected by accuracy modifiers. It also gets Calm Mind at level 25, which makes Gardevoir nearly unstoppable after one or two setups; Speed is the only thing that can prevent it from mass sweeping in some situations. After evolution, Gardevoir learns Psychic at level 33, which is incredibly powerful for this point in the game, especially backed up by Gardevoir's great Special Attack stat. The other level-up moves are much less interesting: there is Imprison at 40 which is just useless, Future Sight at 45 which is outclassed by Psychic in every possible way, Captivate at 53 which is way too situational, Hypnosis at 60 which is unreliable at best, and Dream Eater at 65, if Gardevoir ever get that far, which is useless without Hypnosis.
Gardevoir's already fairly good coverage can be further powered up by massive TM usage. One obvious option is Thunderbolt from the Veilstone City Game Corner, and another great addition is Shadow Ball, which allows Gardevoir to be somewhat secure against some of the special Ghosts, with Lucky Chant and Calm Mind combined. Another good alternative is Focus Blast, though it is very inaccurate and thus probably unreliable. Grass Knot is another valid option on the Grass coverage side. Support sets can run either Reflect or Light Screen, or even both of them, as well as Thunder Wave.
Via tutor, Gardevoir gets access to a few more options, but most of them are physical in nature. The only potentially interesting special one is Icy Wind, but while Ice coverage is good, the low base power of Icy Wind is a fairly big deterrent for its usage.
Calm Mind setup: Calm Mind, Psychic, and any two between Shadow Ball / Thunderbolt / Grass Knot
Steel-types: Steel-types are fortunately easy to come by in Sinnoh, and their defensive coverage spans across all three of the pure Psychic-type weaknesses, which is very convenient for Gardevoir. Physically oriented Steel-types do an even better job, providing the defensive bulk Gardevoir lacks.
Physical tanks: Though Gardevoir can somewhat repair to its lacking Defense with Reflect, the screen move is temporary and can do nothing against critical hits. Since there are several available physical tanks to choose from, it is advised to have at least one on the same team as Gardevoir.
Minimum stats are calculated with 0 EVs, IVs of 0, and a hindering nature, if applicable.
Maximum stats are calculated with 252 EVs, IVs of 31, and a helpful nature, if applicable.
What Nature do I want? Anything that lowers Attack and boosts something else. Modest is probably the best for Calm Mind setup carriers, while Bold and Calm are excellent for support sets. Timid also works well. Speed-lowering natures can work in some cases, but need to be wary of the aforementioned threats. If Ralts has a Special Attack-lowering nature and is male, do not evolve it into Gardevoir; opt for Gallade instead.
Which Ability do I want? Both of them can be useful in some ways. Trace can work wonders against Flint with the due luck, but that is a risky strategy; stall sets will generally prefer Synchronize unless they rely heavily on Toxic or Thunder Wave.
At what point in the game should I be evolved? Right into Kirlia after being caught, before any kind of major battle, and then as soon as possible into Gardevoir, and no later than Maylene. Kirlia's base stats are way lower than Gardevoir's, and it is very easy to lose a Ralts liner before it reaches its last evolutionary stage.
How good is Gardevoir in a Nuzlocke? Quite good, especially for a Psychic-type. Its downside is the substantial lack of Speed in a region where there are a good few fast enemies, and those tend to hit physically, making them off-limits for Gardevoir; however, it is also very capable of facing several other enemies that would be off-limits for other Pokémon. It has its niche, and is very good within its comfort zone.