Next finished miniature!
Rules wise, I play this guy as a Lieutenant (Combi-Melta and Power Axe, as seen on the model) but background wise, he is more like the personal attendant of the Chapter Master.
The empty scabbard on his back is meant to hold the Master's Blade (called "Sarus' Legacy", in case anybody was curious) in case he needs to let go of it for whatever reason. This is a quite honored position within the Chapter, seeing as this makes him as one of the only people who are allowed to carry the blade other than the Master himself.