Captain Katarn Colchis
Katarn, called "Colchis", is a true Colchisian marine. He is completely devoid of creativity or initiative, but 10.000 years of lead men into battle have made company command a reflex for him. He is also very good at teaching new recruits and seasoning young marines (without getting them killed). Unflinchingly loyal to whomever is the current Apostle.
Katarn always fights in a very old suit of Mark II Power Armour. In fact he's never without his armour (in pulbic at least) and will avoid any setting where he can not take it to town. As far as his chaplain can tell he's physically capable of removing his armour - so his refusal to do so is grounded in his (very old) mind.