Marcus definitely has similar political leanings as several of my characters.
I'm not sure they'd naturally like each-other though, which would be interesting, because my charries are still willing to work with people they dislike.
Selwyn
Was part of the Order until he had a breakdown and they had no empathy. He believes they're ineffective at best and create a cycle of violence with the Death Eaters at worst.
Michael Corner
Believes the Order should be formed in emergencies only and the Death Eaters should be left to the aurors to contain. Because of his own experiences with Dumbledore's Army, he also has huge problems with them endorsing Potter's Army. He basically thinks it's like using child soldiers.
Also, Terry, Michael's husband, is an Unspeakable, so Marcus might know him.
Monroe Ashcroft
Monroe is incredibly intelligent yet still has trouble understanding what he considers the subjective categorisation of magics into "light" and "dark".
He definitely wouldn't have any trouble understanding the desire to use "dark" magic toward good ends.
Tifanie Bittel
Tifanie is Caelani Bittel's sister. She's an Unspeakable and she's looking for Calli. If she somehow found out Marcus had been in the Order, especially when it was while Calli was a Death Eater, she'd definitely want to get to know him better.