Chapter Serf
Space Marines are big tough supermen and all, but even they need a helping hand. Someone has to to polish their armor and nob, cook their food, sweep their halls, clean their toilets, file their tax returns, and keep the fortress-monastery defended when the brothers are out stomping Bloodletter faces into the ground and tearing Ork rectums apart with chainswords. These people are Chapter Serfs and live comfortable, well-to-do lives... at least by Imperium of Man standards.
Chapter Serfs (formerly Legion Serfs), were originally picked from failed Space Marine Aspirants and other natives of Space Marine homeworlds who, though physically unable to become Space Marines, still wished to serve. The children of Chapter Serfs also often become chapter serfs themself. Generally speaking, Chapter Serfs are well-trained, highly educated, and treated well; they are officially part of the Chapter. They also have access to highly-sophisticated weaponry (even the Imperial Guard don't get access to this shit). Of course, that does not include Bolters. Not out of elitist exclusivity or anything, but rather because they are heavy and if a mere mortal managed to pull the trigger on one of them it would snap their humerus like a slim jim, however light the recoil.
How well they're treated depends on the Chapter they serve. Ultramarines, Blood Angels, Space Wolves, and Salamanders treat them as equals to other Space Marines, like soldiers might treat their friends in the supply corps. Iron Hands and Imperial Fists and their countless successor chapters see them as just kind of there and don't really care one way or another, the way you treat the fast food workers and garbage people who make your comfy life possible. Marines Malevolent, being dicks, treat them like slaves. Flesh Tearers probably treat them as a walking snack cache. Don't even get us started on how the Legion Serfs in the Traitor Legions are treated (like really bad, imagine being forced to watch erotica berzerker porn with bloody panties and tampons. Don't even think about it!)
In summary they are the Alfred of 40k, because no fine super hero are without a friend/allies that would comfort them after all the grimdark shit happened outside to fight another day.