Okay, I have something like a basic outline for this potential character now:

As a kid, this guy was caught up in some form of Chaos attack that killed... a large proportion of his home planet, probably, but certainly nearly everyone in his home town. He himself lost an arm, and as a result of the excess Warp power in the area, he awakened a fair modicum of psychic power, then inexplicably managed to escape death with his soul and sanity barely intact. "Barely" by 40K standards, of course, meaning he wasn't lost outright to the Ruinous Powers, but still ended up severely mentally scarred. Being on a planet with perhaps slightly over modern-day standards of technology, he found the resources to build himself a new arm, only to become obsessed with making it ever more complex and better able to defend himself from being killed, resulting in an obnoxiously bulky construct with a bunch of automatic weapons and a lot of integration into his flesh to support it, which only got more complicated once he finally found his way off-planet, discovered how comparatively weak the arm actually was, and rebuilt it to the best of his abilities, becoming a mercenary to fund the rebuilds, and because what the hell else could he do in his state? All the while, he was forced to fend off daemons which were drawn to his mind every so often, something that he as a witch just about managed to deal with by manifesting the encounters as horrible hallucinations of monsters in a world of gore and metal rather than the reality of mind-fracturing monsters in a soul-obliterating realm of madness and Emperor-knows-what, so he's naturally not in the best state mentally, but better than he would have been otherwise (totally insane and/or dead and/or possessed). He hardly dares to use the fairly limited psychic powers he has, and unless forced, will do so but sparingly.

I think that incorporates assets from everything that's been brought up as possibilities for him so far other than the Jokaero's digital weapons. Obviously, that's just a rough draft; [@Jbcool] will receive the full version in a while, assuming nothing I've brought up is too infeasible. If anyone else has comments for further improvement, that'd again be appreciated.