Zidane - Zidane is was also a member of Tantalus and was one of Marcus's closest friends.
Blank - Blank is also one of Marcus's closest friends, most likely his best friend. He calls Blank, his "bro" which means that he cares about him very, very, very much!
Cinna - Well, these two don't interact much, but they seem like they are pretty good friends.
Baku - Baku is Marcus's boss. I guess he has a father - son or boss - employee relationship.
Ruby - Ruby has Blank on the end of her little finger, and his best friend, Marcus just watches on, as seen in the section of the game where she opens her play in Alexandria.
Benero, Zenero & Genero - These guys don't interact much with Marcus, but Zenero and Benero did fight with him in the play section of the game.
Quina - I don't think these guys have really met.
Dagger - Dagger and Marcus seem to get along quite well, in fact, Dagger is the one that tries to help out with Marcus. Marcus most likely looks at Dagger as his little cousin, as he referenced her to changing a great deal, then changing his mind again when he realises that she's still a bit naive.
Eiko - These two haven't really met.
Steiner - Steiner thinks of Marcus as he does with all the other Tantalus members. Dirty, rotten theives, which is DEFINATELY not the case. Steiner doesn't want Dagger to hang around with Marcus, so he kind of leads her away from him, and discourages her to go with him. He continiously picks a fight with Marcus, Cinna, Blank and Zidane, and even Baku.
Amarant - These two haven't interacted much, but I think that Amarant doesn't really respect him.
Freija - These two haven't interacted much either.
Vivi - Vivi, as he sees Cinna as his uncle, most likely sees Marcus that way too, "Uncle Marcus" ^^;;