Icarus is Pit's human name who appears as a playable character in Kid Icarus: Severed From the Heavens. He is the brother of Keteus and son of Iapyx. Pit becomes Icarus when his wings are cut off by Amazon Pandora, and can switch between these two "forms" after beating the game and learning a certain skill.
Icarus is basically Pit without wings. Enough said.
Icarus acts similarly to Pit, but is often confused as he cannot remember his past. He is not as knoweledgeable as Pit, as he is without Godly assistance. Icarus also realizes at the end of the game how much harder the humans must work than angels, and grows to appreciate them more.
Icarus is not as good at fighting as Pit, as he cannot remember training with Palutena's army. However, his skills do improve over the course of the game, as he is mentored by Magnus for much of it. Icarus has access to a wide variety of magical powers and can earn more as he progresses through the game.