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. When playing a two-player game, player two plays as Nana. In both Melee and Brawl, certain color changes will make Nana the leading, player-controlled climber.
In Super Smash Bros., 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. The payoff can be highly rewarding, however; they are placed near the top of most accepted tier lists.
Both of the Ice Climbers are small, young children. They both wield wooden mallets, which they use to whack enemies and break ice cubes. Both of them wear parkas; Popo's parka is blue while Nana's is pink. They can also be identified by the patch of hair exposed from their hoods.
The relationship between Popo and Nana is unknown and was not elaborated on in Ice Climber. As the two are said to be "rarely seen apart" in Melee, among the ideas thought up of by fans are that the two are siblings, friends, or even lovers. According to Nintendo, the nature of their relationship is up to the player's imagination; as a result, no hints to their relationship has been directly stated in any media. Melee offers some insight to the potential relationship to the two; a brief shot in the Opening of Melee features Nana and Popo high-fiving each other, with a small heart coming out from where their hands meet, and the Japanese title of Event 21: Ice Breaker roughly translating to "Be the jammer of their romance".
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 were selected over Balloon Fighter from Balloon Fight, the main character from Urban Champion, Bubbles from Clu Clu Land, and the Excitebiker from Excitebike, to be representatives of the Famicom/NES period in Nintendo's history.
Main article: List of SSBM trophies (Ice Climber series)
Main article: Ice Climbers (SSBB)
The Ice Climbers still consist of Popo and Nana. They still fight as a pair with Nana following along. 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 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 claimed 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 were 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)