This one began life as a chaos cultist. All the extra bits (arms, head, backpack) are from the various mechanicus kits.
Once you get them painted, they look distinctive enough.
But a certain resemblance remains. I'll have to run a scenario (or campaign). Brothers from the same vat, grown to be labourers on an agri-planet, inseparable friends. Until a childhood accident changes their lives forever. A beserk pest-sprayer bot attacks the boys, mangling one of them, and dousing both of them with highly toxic pesticides. Both boys suffer chronic lung problems.
One brother rages against the very fabric of the Imperium. 'What society would let a man work and die in these conditions?' The twisted path of chaos seems a far more rational and humane way of life (?!) Meanwhile the other brother channels his fury into endless slo-mo training montages, painstakingly rebuilding his strength and honing his combat skills. He dedicates his life to tracking and destroying rogue technology.