Clefairy is a very rare encounter in Mt. Moon, with a 1% encounter rate on the upper floors and a 6% encounter rate on the lower floors. It can also be bought for 500 coins at the Celadon Game Corner in FireRed and for 750 coins at the Celadon Game Corner in LeafGreen.
Moon Stones are available in Mt. Moon, the Rocket Hideout, and the Pokémon Mansion. They are also held by wild Clefairy.
Protip: If you’re dead-set on getting a Clefairy and using it in a Nuzlocke, you don’t have to wait until Celadon City. Buy a Repel or two and head to the second basement floor of Mt. Moon (B2F). The odds are still pretty slim, but it’s worth a shot if you want to use this pink blob and its amazing coverage.
Clefairy’s versatile stats allow it to fill a variety of roles, such as physical attacker, special attacker, and (in a pinch) tank. Like most Normal-types, Clefairy can learn a wide variety of moves. Even better, Clefable’s attacking stats actually allow it to use its movepool - which is more than can be said for Raticate. In Gen I, this gave the pink fairy the ability to solo the E4 (!) with a moveset of Body Slam, Ice Beam, Psychic, and Thunderbolt, so long as it was boosted with X Specials at the start of every battle.
Unfortunately, Clefairy’s rarity prevents it from being seen in most Nuzlockes. Most gift-Pokémon rules force you to get only one Pokémon from Celadon, making you have to choose between buying Clefairy (or another Pokémon) at the Game Corner and obtaining the versatile and highly-rated Eevee. (This assumes you miss the extremely rare Mt. Moon catch - a reasonable assumption.) Given the choice between adding a potent Water-type, Electric-type, or Fire-type to their arsenal and getting yet another Normal-type, typically players will choose the former.
Clefairy’s main disadvantage in battle is its low speed: it almost always ends up taking a hit before retaliating. Its fully evolved form also isn’t as bulky as conventional tanks, with 95 HP, 73 Defense, and 90 Special Defense. Luckily, its bulk is still sufficient to allow it to take lots of hits and fight back hard, and techniques such as Minimize and Defense Curl allow it to enhance its natural durability to impressive levels. Overall, the Clefairy line is a great replacement for your Normal-Type “derp” with its all-around stats and diverse movepool.
Rival (Cerulean City): If you’re lucky enough to have Clefairy by this point, make sure to teach it Mega Punch on your way out of Mt. Moon. It will punch holes through Blue’s team as Clefairy shrugs off his team’s attacks. In fact, if you want, you can even have a fully-evolved Clefable by now, since Clefairy really doesn’t get any good moves until Cosmic Power (level 33), and Clefable’s stats absolutely wreck this portion of the game, especially in tandem with STAB Mega Punch.
Gym #2 - Misty (Cerulean City, Water-type): You may have evolved Clefairy into Clefable by now. If so, Clefable can easily tank Water Pulses and fight back strong with Mega Punch. If you have no Ivysaur/Bellsprout/Oddish, Clefable is a great choice for this gym battle. Clefairy does alright against Staryu, but will fall short against Starmie.
Rival (S.S. Anne): Clefable still has nothing to fear due to its great bulk for this point in the game. You can pummel his team with Mega Punch while barely feeling the attacks launched at Clefable. Clefairy, on the other hand, struggles against the physical hitters, notably Pidgeotto and Raticate, though it can hold its own against the special ones.
Gym #3 - Lt. Surge (Vermillion City, Electric-type): Clefairy’s Special Defense stat will allow it to withstand a couple Electric attacks, but you are better off using Diglett. Clefairy and Clefable do learn Dig via TM28, though, so you can try using that if you want.
Rival (Pokémon Tower): Gyarados’s Thrash threatens Clefable, but teaching Clefable Thunderbolt before the battle will enable it to one-shot the overgrown carp. Aside from that, continue as normal with Mega Punch or a similarly strong move. If you still have a Clefairy, only take on the special attackers and maybe Growlithe.
Giovanni (Rocket Hideout, Ground-type): If you lack a Water-type, teach Clefable Water Pulse and proceed to rampage through his team. Kangaskhan presents a problem with its physical bulk, but two can play at that game - answer with Mega Punch or super-effective Brick Break. Just be careful if your HP gets low. Clefairy can take down Onix and Rhyhorn with Water Pulse too, but Kangaskhan is off-limits for it.
Gym #4 - Erika (Celadon City, Grass-type): Grab Flamethrower/Ice Beam from the Game Corner and torch/freeze the place. Ice Beam is a better choice in the long term due to Lance’s team. Again, if you still have a Clefairy, tread with caution because Erika's sweepers are deceptively strong.
Gym #5 - Koga (Fuchsia City, Poison-type): You should have a Clefable by now, even if you waited for Cosmic Power. Go to Saffron, grab the Psychic TM from Mr. Psychic, and sweep. By this point, if you still have Mega Punch and you want to retain Normal STAB, Strength has the same base power and has 100% accuracy. You can also try Return for the extra power, but since it’s only obtainable once, you might want to use it on someone else.
Fighting Dojo (Saffron City, Fighting-type): Risky players may want to attempt a Psychic sweep, but it’s generally a poor idea to send out a SLOW pure Normal-type against a Fighting gym. For obvious reasons. I mean, come on.
Rival (Silph Co.): Thunderbolt Gyarados and Pidgeot, dispatch Growlithe and Exeggcute with the move of your choice, and use super-effective moves against the starters. Be wary now that they’re in their fully-evolved forms. Flamethrower/Ice Beam/Thunderbolt/STAB move pretty much destroys anything your rival can throw at you, so if you have enough money and/or patience, go grab some of those TMs from the Celadon Game Corner.
Giovanni (Silph Co., Ground-type): Use Psychic against the Nidoran lines and strong special attacks against the rest. Remember that Ground-types are immune to Thunderbolt.
Gym #6 - Sabrina (Saffron City, Psychic-type): Careful here - those Psychics are very powerful. Return and Strength both hit hard against their poor Defense, though Clefable is likely unable to afford fighting the whole team on its own.
Gym #7 - Blaine (Cinnabar Island, Fire-type): Use Water Pulse if you still have it. Otherwise, consider letting Clefable sit out the battle; you have a Surfer, use that instead. In a pinch, Clefable can still switch in and finish up with its Normal STAB or Dig.
Gym #8 - Giovanni (Viridian City, Ground-type): Water Pulse if you still have it, though Psychic is preferred against the Nidoran line. Beware of Dugtrio, which will outspeed and deal massive damage with Earthquake before dying.
Rival (Route 22, pre-Elite Four): Thunderbolt the Water and/or Flying types. Flamethrower or Ice Beam the Grass-types. Alakazam is possible with strong physical attacks, but beware of Psychic. Chances are, you can tank at least two non-crit Psychics, but be careful of weird damage variance and critical hits, as well as Special Defense drops caused by Psychic. Handle the unevolved Pokémon with the move of your choice. Be careful of the starters: they’re carrying powerful STAB moves, and Clefable’s bulk isn’t sufficient to let it take them with impunity.
Elite Four Lorelei (Indigo Plateau, Ice-type): Thunderbolt, Thunderbolt, Thunderbolt, Thunderbolt. Oh, and hit Jynx with Flamethrower or strong Normal STAB. Lapras probably isn’t going down in one hit, but can’t do too much back to you, as Clefable is quite a capable tank.
Elite Four Bruno (Indigo Plateau, Fighting-type): Possible, but you’re playing with your Clefable’s life here. If you must, use his Onix as time to boost up with X Speeds, and X Specials, if you want to be cautious, or Calm Mind if you have that. You NEED either Cosmic Power or to OHKO the Fighting-types to pull this off. In the Cosmic Power case, remember that critical hits will still do you in.
Elite Four Agatha (Indigo Plateau, Poison-type): Psychic, Psychic, Psychic, Psychic, Psychic. Next? (You may wish to bring Elixirs or Ethers to the League if you’re relying on Clefable. It’s an excellent Pokémon, but only has so much PP.)
Elite Four Lance (Indigo Plateau, Dragon-type): Thunderbolt, Ice Beam, Ice Beam, Ice Beam, Ice Beam. Again, next? (Here, bear in mind that Aerodactyl and Dragonite may live a hit. Be prepared to take one yourself.)
Champion Rival (Indigo Plateau): Thunderbolt, Ice Beam, and Psychic will serve you VERY well in this battle. As with Bruno, however, you may want to use his weak first Pokémon to boost up with X Speeds and X Specials - he has no Fighting-type attacks on his team, but his Pokémon’s STAB moves hit exceptionally hard.
You should start off with the moves Pound, Growl, Encore, and Sing. Pound is a mediocre STAB move, but it will do at first. Growl can be useful on some occasions, as Clefairy’s physical bulk could use some help. Sing is incredibly useful if it hits, but 55% accuracy makes it as much a hindrance as a help. If you miss more than two in a row, that’s probably a sign that you should stop trying and just get on with trying to KO the opponent. DoubleSlap comes in at level 13, but it’s rather unreliable - stick with Pound. Follow Me at level 17 is useless. Minimize, at level 21, is a pleasantly evil move, but relying on hax in a Nuzlocke is a good way to get your Pokémon killed. If you use it at all, use it at the start of a battle in which you intend to have Clefairy/Clefable sweep all Pokémon, so that you’ll get the most mileage out of the raised evasion chance. Level 25 brings Defense Curl, which is decidedly meh. Metronome, best known for getting Pokémon killed through pulling Explosions, comes at level 29. Continuing to level 33 brings Cosmic Power, which is excellent for setting Clefairy or Clefable up to laugh off everything but critical hits. Moonlight comes in at level 37, though you won’t get much mileage out of it unless you’re doing a restrict-healing run. Level 41 brings Light Screen, which is handy for protecting Clefairy or Clefable against physical attackers. Finally, level 45 gives Clefairy Meteor Mash, a very powerful move infamous for missing at important moments. Also, despite its high power, its Steel-type doesn’t provide much coverage at all, and its low 85% accuracy isn’t exactly reliable. It’s better to rely on the less powerful but all-around more useful Brick Break, which you can get via TM.
Far more important, however, is Clefable’s TM and tutor movepool. Focus Punch and Substitute, as always, combine quite well, Hyper Beam can add some extra power (though Clefable is probably better off using both turns to slug away with Strength or Return), and Calm Mind has the potential to boost Clefable to truly vicious levels. As for attacking moves, Thunderbolt gives Clefable the ability to take out Water-types and Flying-types, Ice Beam defeats Grass-types and Lance’s infamous Dragons, and Psychic mows through Fighting-types and the many Poison-types of Kanto. Flamethrower provides an alternative attacking option, but Ice Beam is useful against more of the foes in the important match-ups for the same price. Water Pulse is a good move for the early game, when few specially-based TMs are available, but should be replaced later on. Reflect and Light Screen are options for a defensive Clefable. Mega Punch is a good option for the early game, but later becomes outclassed by Strength and Return, while Double-Edge is a potent attacking option, but Clefable will not appreciate the recoil. Brick Break and Dig also provide nice coverage, with the former being arguably the more useful of the two.
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.
Clefairy Line Ratings
What Nature do I want? If you plan to make Clefable Specially-based, obviously you should prefer Special Attack-boosting natures, and Attack-lowering natures are acceptable. Unfortunately, Clefable’s rounded stats make it not particularly specialized in anything, so all stats are bad to lower. Be careful of its dump stat, whichever it is.
At what point in the game should I be evolved? You can potentially evolve it the moment you get it. If you enjoy hax, you may wish to hold off on evolving until level 21, when it gets Minimize. Other than that, its movepool is mostly TM-based, so there’s no benefit in delaying.
How good is the Clefairy line in a Nuzlocke? Assuming you can get it (or choose it over Eevee), it's very good. It requires heavy TM investment to reach its full potential, but its versatility then allows it to take on most foes with aplomb. With skill (or aid from X items), it can take out a large chunk of the Elite Four on its own!
Neutralities: Bug, Dark, Dragon, Electric, Fire, Flying, Ghost, Ground, Ice, Normal, Poison, Psychic, Rock, Steel, Water