Hey! I love Marcus and am tickled to see him in play.
Gemma: would know of him and maybe vice-versa. THey look to be the same age but were in different years, both Slytherins, but she's half-blooded. If Marcus enjoys the less legal side of things, him and Gemma might have some things in common.
Percy: Knows of Marcus from quidditch and has liely heard far too much about him from Oliver ;) Has Marcus ever done anything that would be worthy of Percy's point-taking? I always imagined great animosity between them, but am up for whatever plottage you're up for.
Ernie: Would be polite, but as long as that tattoo is there, Ernie would totally always be on guard around him, just in case.