Ice Climbers

Nana and Popo. They'll climb anything they can think of; icy mountains, tall towers, poisonous animals, and even boats. Popo says his bedroom is in the basement and his kitchen is on the second floor, just so that he can climb something every morning. Nana parks her car on the roof.

There is no true, solid statement on the nature of their relationship in any game. They could be siblings. They could be uncle and niece. They could be doppelgangers from parallel realities. But Masahiro Sakurai said he pictured them as lovers when he developed Smash Bros. Melee, and I think that's the best way to go.