Kingdom Hearts II's Sora in Borderlands 2: his own command menu, his magic and his art, read from your copy of the game.
Kingdom Hearts II Final Mix's Sora is the Vault Hunter. Play Borderlands 2 with his own Kingdom Hearts II command menu sitting over the frame: his HP and MP gauges, and his command bar carrying Attack, Fire, Blizzard, Thunder and Curaga.
You pick Zer0's portrait at character select and he is Sora now. X and Z step the menu, C casts what it is showing. Fire (incendiary), Blizzard (ice-cold force and a hard shove) and Thunder (shock) land on Borderlands 2's own enemies through its own damage types, and Curaga heals you with Kingdom Hearts II's own price in MP - Fire 20, Blizzard 20, Thunder 25, Curaga 60 - paid from Sora's MP gauge, which refills as it does in Kingdom Hearts II.
The menu is Kingdom Hearts II's own art and it is read out of your installed copy of KINGDOM HEARTS -HD 1.5+2.5 ReMIX- while you play: each world has its own command sheet - Twilight Town, Agrabah, Hollow Bastion, the World That Never Was - and the mashup draws the sheet of the world you are in, swapping as you move through Borderlands 2's maps. The command's own icon also comes up large in the middle of the screen when you cast, the way Kingdom Hearts II announces a spell.
Nothing from Kingdom Hearts is included: no art, no sound, no text, no data. Your menu is built out of your own copy of the game, on your PC, every session.
You need Borderlands 2 (Steam) and KINGDOM HEARTS -HD 1.5+2.5 ReMIX- (Steam), both installed. Melty installs the mashup into Borderlands 2 for you - the Kingdom Hearts side is only ever read from.
Solo. Sora is your Vault Hunter on your own run; there is no co-op.
Licence: MIT - remix it, the recipe and the sheets are in the release.