Normally, players first encounter the fighting marsupial at the end of the first section of the 2nd level. He appears alongside an evasive whip-wielding clown who periodically whips him to keep him in line. If this clown is beaten before Roo is, then the kangaroo leaves the area peacefully, and the next time a player loses all their lives, they can opt to play as him.
Roo can an also be rendered playable at the game's beginning if Up & B are held at the title screen, followed by START.
Roo's short, glove-clad arms give him a terrible reach, and leave him unable to use weapons, however, he can strike enemies with his tail for a bit of range, and the height and reach of his jumps are incredible.