Let's see... sticking with non-licensed characters (meaning, not putting in Disney characters since they were movie characters BEFORE Kingdom Hearts)... if I had to summon a game character for reasons other than sex, I would summon:
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Travis Touchdown, since I've always wanted to play video games head-on with an actual game character. Also, since he breaks the fourth wall quite often, he'd probably be the least disoriented by the transfer to this world. Actually, I might volunteer to take his place in the NMH-verse for a while, just so I could PERSONALLY strangle Jasper #@$%ing Batt Junior.
Kefka (pre-God-mode, obviously, since I'm not suicidal), since it would be kickass to crash some parties with him. Especially parties of people I don't like.
Otto from Berserk, since when Xpwn3ntial summons Kratos, I'll need SOMETHING ridiculously unbalanced in order to stop him (and Teddy Roosevelt isn't a game character... yet).
Kane & Lynch, just to get the satisfaction of locking those two douchebags in a room with Travis Touchdown for about 30 seconds, and picking up their organs from the floor afterward. Come to think of it, there's a number of game characters I'd like to lock in a room with Travis and/or Kefka for a while... the entire cast of Gears of War, for instance.
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Now, for characters I'd conjure for sex... well, some obvious choices like Rikku from FFX-2 and Kasumi from DoA... and since I'm bi, I might also take Balthier from FFXII. Oh wait, I have to limit myself to just one...