The Ice Climbers (アイスクライマー, Ice Climber) are the pair of playable protagonists from the game Ice Climber, released in the pre-Super Mario Bros. era of NES games. When playing a one-player game, the player plays as Popo (ポポ, Popo). When playing a two-player game, player two plays as Nana (ナナ, Nana). In the Super Smash Bros. series, the player controls Popo by default, and certain alternate costumes will make Nana the leading, player-controlled climber. The other climber is CPU-controlled and follows the leader in both movement and attacks.
In Smash, the CPU-controlled Ice Climber has his or her own separate percentage or stamina counter, though it is not displayed on-screen. This factor alone contributes to the Ice Climbers' extremely high learning curve, as players must ensure the safety of not one, but two characters at the same time.
Ice Climber was one of the first NES games that allowed for two people to play simultaneously. Its playable characters are two climbers wearing color-coded parkas; a boy in blue named Popo, and a girl in pink named Nana. The Ice Climbers are depicted with cute, super-deformed proportions, and in their original game, it is ambiguous whether they are children or adults. They both wield wooden mallets, which they use to whack enemies and break ice cubes. Since hardware limitations originally limited the climbers to simple 8-bit sprites, their appearance in Melee gave them more elaborate designs, sporting eyes and blushes similar to Kirby. The two were also given unique hairstyles so that they can be distinguished within their alternate costumes.
The relationship between Popo and Nana is unknown, and was not elaborated on in Ice Climber. Their Melee trophy mentions that they are "rarely seen apart", and in their codec conversation, Mei Ling says that the Ice Climbers have a "blood bond"—a bond so unfathomably strong that they would die for each other. Regardless, the exact nature of their relationship has never been directly stated in any media. Some insight can be gleaned through related material; a brief shot in the opening of Melee features Nana and Popo high-fiving each other, with a small heart emerging from where their hands meet, and the Japanese title of Event 21: Ice Breaker roughly translates to "Those Who Interfere People's Romance". The Japanese homepage for Melee, Smabura-Ken, refers to them as "more than friends and less than lovers".
As playable characters
Main article: Ice Climbers (SSBM)
The Ice Climbers made their debut appearance in the Super Smash Bros. series as a dual-controlled pair of fighters. They (as a singular entity) are one of the 14 (15 if Sheik is counted) default characters available from the start of the game. They are ranked 7th on the tier list, in the B tier. They are infamously known in Melee for a strategy called "Wobbling," in which an Ice Climbers player can keep an opponent grabbed while dealing damage infinitely, leading to a single grab being an instant K.O.
They were selected over Balloon Fighter from Balloon Fight, the main character from Urban Champion, Bubbles from Clu Clu Land, and the Biker from Excitebike, to be representatives of the Famicom/NES period in Nintendo's history.
Main article: List of SSBM trophies (Ice Climber series)
As playable characters
Main article: Ice Climbers (SSBB)
The Ice Climbers return in Brawl, in which they once again act as a two-in-one fighter. Both Ice Climbers can now hang on a ledge at the same time and their down special in the air is no longer an auto-freeze. The Ice Climbers' Final Smash, Iceberg, consists of a large iceberg taking up a large portion of the screen. It deals damage and can freeze those on or near it. Players can still choose between Popo or Nana as the leader of the duo depending on which color of costume is chosen. The threat of the Ice Climbers' 0-death chain grabs, together with the game's random tripping, puts them in 2nd on the Brawl tier list in S tier, only behind Meta Knight.
The Ice Climbers do not return as playable characters for Super Smash Bros. 4, though they do make a cameo as a collectible trophy. In a column for the Japanese gaming magazine Famitsu, Masahiro Sakurai stated that technical limitations of the Nintendo 3DS prevented their inclusion; the two were reportedly difficult to implement on the system, and since the characters were unlikely to have a new game in their series, Sakurai deemed the Ice Climbers low-priority characters. While they would have been fully functional on the Wii U, Sakurai decided to cut them in order to keep the playable roster between the games identical. 
Main article: List of SSB4 trophies (Others)
As playable characters
Main article: Ice Climbers (SSBU)
The Ice Climbers return as playable characters in Super Smash Bros. Ultimate, after having been absent from Super Smash Bros. 4. Unlike their previous appearances in the series, they are unlockable characters. The Ice Climbers in Ultimate are considered to be nerfed, as they can no longer wobble or chaingrab, in addition to having relatively poor mobility and range. However, they can still perform very complicated desync combos with very situational setups, which can lead to a K.O. relatively early.
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