Hmm...probably wouldn't be a full tail...that wouldn't really work so well since it wouldn't be flexible...but perhaps a tail-blade modification? Your character maybe cracked or broke his tail-blade somehow in his past, and then had it repaired with an alloyed, partially-prosthetic tail-blade? His entire tail would look normal until you got to the blade which has been modified and enhanced? He'd probably use a better alloy than steel though, I would assume. Just a few ideas to mull around. But if you are going to have your character have an implant(s), then he'd have to be someone either close to Iksilon (like a member of his personal squad), or another important/high-ranking member of Sector Seven. The implants are kept on a very strict "high-rank/need-only" basis. Also, keep in mind that a prosthetic that large wouldn't be bio-adaptable, meaning that if he morphed, he'd end up leaving the prosthetic behind.