Marcus

The 'Facts':

He joined the NLM, went to Quaquan, saved some lives then later collapsed in a hallway. We surmise it's cause he's an alcoholic too, like Ace but with a penchant for Peach Schnapps.

The Truth Of The Matter:

Having seen Jeff Ryan in passing one day, Marcus felt a flutter at his heart strings. It didn't matter that Jeff was (supposedly) happily married, so long as this pining doctor could be near him. The next logical step was to join the NLM as a Doctor, which thankfully he wasn't just pretending to be. At least in this way he could be close to his one true love and probably get a little feely feely going on when Jeffy was drugged and no one else was looking.

Rumour has it that after a brief struggle he got his chance.