I would be an extremely fast character. My speed is basically to the point of teleportation. In short, I'd be able to move faster than the naked eye can see from a standing position and I can stop on the head of a pin (since a dime is too big). I'd also carry a dagger, with the ability to drain the life of the hero and his party (if there is a party) and a greatsword. While the greatsword would seem to be the greater threat, since its larger weight doesn't really affect my speed, the dagger is actually the prime threat since I'd use it whenever I get to about 50% hp to fully heal myself via superspeed slashes on the hero(s).
The only way to beat me would be to somehow nullify my speed by nonmagic means (since my speed would be nonmagical and therefore not affected by anti-magic). This speed would also give me some pretty cool acrobatic abilities basically making every area of the boss room, or area if outside, reachable (flying archers won't be untouchable to me).
Normally I wouldn't use my full speed until the hero hit me for the first time or until I see that someone in the group has AoE (AoE users would be the first target since they'd normally be fairly weak, physically, and I could easily cut them down before they knew what would happen). Also, any cheatcodes the player would use would also be attributed to me, which means any codes that give them invincibility give me invincibility. And since the hero has to live through to the cutscene any codes that would kill them would just waste their time (no suicide and hope for the cutscene).
If I had to choose a play style for this game I'd have to say a third person RPG that would have a combat system similar to a combination of Devil May Cry (ramped up a few exponential values) and Infinite Undiscovery (Camera angle and party system). Also, I'd like the aim of the game to be more on a focus of being a great hero and not some sort of crusade against evil. If the game were like this, then my character could be the lazy yet reliable guy that just seems to hang about and do nothing, this does not mean his deeds and ability are unknown.
Also, upon winning, I would simply wave the hero(s) off and go back to sleep wherever I was. If they somehow win, then I would basically do the same thing except say good job (Some larger organization would take care of the final cutscene).