Mochi can usually be seen wandering around the Professor's laboratory, eating, sleeping or washing himself while the Professor occasionally talks to him. He will sleep on top of Terry's bed if Terry is knocked out by an enemy and returned to his room, but will run off back to the lab before Terry wakes up.
Whenever Terry takes a nap and saves the game, the player will be able to play with Mochi for a brief time in the laboratory. Using the stylus to direct him to his food bowl, scratching post and so on will make him happy, while touching him directly will annoy him. Moving the stylus in a small circle around Mochi will make him roll over onto his back and let the player pet him directly.
Mochi can be summoned at any time by using the Mochi Decal; applying it to the screen will make Mochi appear and run around while silly music plays. Depending on his level, he will either wash himself, amble about and then run off, doing a single point of damage (or sometimes no damage at all) to enemies, stun opponents or even do comparatively high damage to all creatures on screen. Playing repeatedly with Mochi and making him affectionate towards the player will increase Mochi's level and the damage he does when summoned.