Capri's origin is unknown and since he wanders freely from universe to universe, no one even knows which universe he came from originally. He looks for entertainment but is easily bored. In fact, since he is nearly omnipotent, usually the best way to defeat him is to bore him until he wanders off looking for someone new to play with.
He isn't always bad. He rarely causes harm intentionally. His most typical game is to kidnap people and let them loose in new environments. For example, he'll take someone who is used to technology and strand them on a primitive planet or he'll take someone who is used to a patriarchy and strand them on a matriarchical planet. The biggest problem is that he doesn't return people to their home even after he's grown bored with them.
The reason why he's listed as part of Earth 49996 is not because he spends much time there but because the Magistrates (Earth-49996) have listed him as one of their highest priorities. They monitor his actions and when they can, they rescue his victims.
He is nearly omnipotent with major reality bending powers but he rarely uses these powers unless threatened. He does not age and is nearly immortal but he can be killed. However, the few beings powerful enough to kill him do not want him dead and have even protected him for their own mysterious reasons. The most frequent power he uses is teleportation.