Popo is in front, and Nana is in the back
The Ice Climbers are a pair of mountain climbers consisting of Popo and Nana. They appear in Ice Climber, Super Smash Bros. Melee and Super Smash Bros. Brawl. Popo also makes an appearance in one of the microgames in WarioWare Inc: Mega Microgame$!. They are unique for being the only character that consists of two people in the Super Smash Bros. series. In the Super Smash Bros. games, the leader of the two will always be Popo, but if the right costume is changed, Nana will appear to be leading as Popo will be in the back wearing light blue or white, and Nana will be in front wearing orange or red. This matters because if the leader is defeated first, then the follower will automatically lose. Though if the follower is defeated first, then the leader may continue to fight by his or herself, albeit with a distinct disadvantage.
Also in Super Smash Bros. Melee and Super Smash Bros. Brawl, With Nana by your side, your specials will do twice damage. Without her the Belay Move won't work, so do whatever is needed to save her, even if she is in a very dangerous situation. If she is KO'd, Popo will remain alive, but if Popo is KO'd, Nana will be auto KO'd.
The Ice Climbers are set to be returning characters in Super Smash Bros. Ultimate, evidently working much like they did in Melee and Brawl.
The ice climbers are young but there age is unknown. They only are known for 3 games the first being ice climber and there last two games being super smash bros. melee and brawl it is unknown if they will star in any other new Nintendo games.