According to Wikipedia,
Fantomex has an external nervous system referred to as E.V.A. He has mentioned that E.V.A. emerged from his mouth during his time in the World and developed into a techno-organic flying saucer-like vessel. The relationship between E.V.A and Fantomex is symbiotic, so if E.V.A. experiences pain when she is not within Fantomex, he will feel it as well. When E.V.A. is separated from his body, Fantomex feels no pain and seems able to ignore most injuries.
I've been reading Uncanny X-Force and it's left me scratching my head with questions like what E.V.A. is, where it/she resides when not in ship form, etc. While the above explains some of it, it doesn't provide a reference to which comic E.V.A.'s nature is explained in.
Is E.V.A.'s nature conclusively explained in the comics, and if so, where? Any additional information about E.V.A. would be appreciated as well.