The most overpowered usable keyblade wearer in any Kingdom Hearts game 
Aqua FTW
Also, she is no way useless, storywise. Had anyone played the Final Chapter, you would know why her, on par with Ven, are the most important characters for the development of why Castle Oblivion is at is, thus affecting the entire storyline post-KH1.
None of the three keyblade masters is useless. Heck, all the KH lore revolves around them.
Current PC Build
CPU - i7 8700K 3.7 GHz (4.7 GHz turbo) 6 cores OC'd to 5.2 GHz with Watercooling (Hydro Series H110i) | MB - Gigabyte Z370 HD3P ATX | Gigabyte GTX 1080ti Gaming OC BLACK 11G (1657 MHz Boost Core / 11010 MHz Memory) | RAM - Corsair DIMM 32GB DDR4, 2400 MHz | PSU - Corsair CX650M (80+ Bronze) 650W | Audio - Asus Essence STX II 7.1 | Monitor - Samsung U28E590D 4K UHD, Freesync, 1 ms, 60 Hz, 28"
