Starfan13 and Star are both selfinserts, but Marco is by far not Bobby (Daron's husband), let me explain.
Starfan13 is Daron reacting as a girl to Star who was created as a selfinsert of Daron, how she imagined her life would be if she had magic (explained by herself in the different interviews she had in 2014 and 2015 before and post the premiere in January).
Marco on the other hand was created very early on her "project" that was Star & the forces of evil, he was created based on sole crush she had as a girl and was more of an enemy than a friend.
Apparently said crush had his own crush over one of Daron's friends, the thicc stacy at Daron's left, in the pic we can see a girl with that could be Kelly, Janna and even Brittney (the hair is quite an indicative as it's all curly and shit) they are all back there.
So yeah, Marco is not Bobby is the guy she never got over as he didn't give a fuck about her and never knew about her feelings.