Super Smash Bros. Brawl is a dream Snake has in the last few hours of his life as he lays catatonic and dying after MGS4. All of the characters are analogies of people he's met, things he's witnessed at some point, or symbolic of his ideals.
Fox, Wolf and Falco are representative of FOXHOUND's codename system as well as Gray Fox, Sniper Wolf and Vulcan Raven respectively. Ike is a representation of the mercenary lifestyle he had prior to MG1. Luigi symbolizes his role as second best in the shadow of Big Boss. Mario and Yoshi are the action figures he saw in Otacon's lab in MGS1. Sonic represents the rivalry between hedgehogs and snakes because snakes are consumed by them. Ness and Lucas are figurative representations of his childhood; Lucas lived a life full of tragedy and without a family, eventually ending up having to fight his twin brother, while Ness had an ideal childhood Snake wished for -- a loving mother, friends, and the admiration of everyone. The Pokemon represent his role as a tool of war and the concept of being bred to fight in a life of conflict, and the Pokemon Trainer is a metaphor for child soldiers.
Pikmin go hand-in-hand with the Pokemon. They're tools used by a force that could not care less if they die because they're expendable. Ice Climbers are a metaphor for the years of his life spent in Alaska. Mr. Game and Watch is Snake's representation of how he sees Otacon, a flat, two dimensional creature of data.
Link is a metaphor for the legendary hero people see Snake as. Pit is symbolic of the role he had in FOXHOUND before Outer Heaven: a loyal soldier, completely trusting his superior (Big Boss) until the end. Zelda and Peach are symbolic of the women in distress Snake has had to save on occasion and their position of importance, and Samus represents what his ideal woman would be. The villain characters--Ganondorf, King Dedede, Bowser--represent the many outlandish but powerful enemies he's had to fight in the past, and Master Hand, Crazy Hand and Taboo represent the Patriots.
Yes, I did steal this.
Pixel Art can be fun.
