I feared the worst when the Mad Doctor put me on this thing.
–Gremlin Calvin, Epic Mickey
Gremlin Calvin is a greenGremlin found in Epic Mickey, first appearing in the Dark Beauty Castle level. Here, he's trapped in a cage on a catapult, and Mickey has a chance to rescue him. Or, if the player decides otherwise, there is a treasure chest containing 200 E-Tickets in the form of whites sitting on a pressure panel that, when disturbed, will launch Calvin away. If Mickey decides to take the heroic route and break the cage holding Gremlin Calvin, then he will lock the panel away, and thus Mickey will not get the tickets, but will reward Mickey with a Gold pin. If Mickey gets the tickets instead, Calvin is launched, much to Gus' dismay.
Later, when Mickey reaches Gremlin Village, Calvin appears near the Gremlin Village Windmill if Mickey freed him. He rewards Mickey with a green E-ticket, which is worth 50 E-tickets. But if he was launched, it is unknown what happens to him, since he is never seen again.
When Calvin reappears in Gremlin Village, he has somehow become a Green Gremlin.
There is a glitch with Calvin. Sometimes, even if you rescue him, he can be missing. He possibly fell off to the pool of Thinner underneath.
If the game is hacked so that Mickey has an Anvil Sketch at the start of the game, he can drop one on the pressure plate to break the chest and collect the contents, then rescue Calvin. Calvin will congratulate Mickey, but then the catapult animation will play and Gus will scold him, curiously with Calvin safely nearby.
Once Mickey reaches the Gremlin Village, Calvin is still found next to the Windmill, so it's likely that Mickey still follows the good path despite Gus's reaction.
According to the PC files, Calvin is the Gremlin who is captured in the Train Dioramas in Epic Mickey 2: The Power of Two.