Erementar Gerad

Coud is a young sky pirate who discovered a girl with mysterious powers on what had seemed like a routine mission. Ren has the ability to lend Coud bizarre fighting capabilities (his arm turns into a giant blade of some sort), being part of a race of beings called Edel Reid, but being the possessor of a stone called the Elemental Gelade, she has powers even beyond the norm.

(Source: Tokyopop)