Marco is played by Jerry and his PB is Jay Hernandez.  He's from Animorphs.  For more information than the lazy mun can type about him, see his wikipedia entry here:  http://en.wikipedia.org/wiki/Marco_(Animorphs)

Now.  The way I play Marco is that he often wears a mask.  There's some pretty obvious back-up for this in canon.  Basically, he'll whine and moan about small shit, like the pizza guy being a little late, the beer being too flat (although the mun agrees that this is a legitimate complaint) and, "Jesus fuck Ultimates 3 what the fuck?"  But the big shit?  Losing people he cares about, his dark side, that sort of shit?  You won't hear one little peep about that until his internalization has reached critical mass and he fucks up his life somehow.

I also play him as having some serious unrequited feelings for Rachel.  He clearly had a crush on her in the early days, and while the general interpretation of their relationship is that it sort of stopped, mine is different.  Marco, being a very analytical, calculating person looked at the facts, realized he didn't stand a chance, and just kept his damn mouth shut.  Wiki claims he was the least affected by Rachel's death, to which I say, "Duh."  He doesn't talk about his big problems.  He rarely talks about his feelings.  He's not going to go off on some self-imposed exile, he's not going to collapse in on himself, or any other such, in his words, "emo bullshit".  He grits his teeth, shoves it all in a deep dark hole, and tries to ignore it until finally all of that stuff mutates into a giant monster and threatens to rip him apart from the inside out.  So?  He did this with Rachel's death.  He mourns, he stuffs the loss and the pain inside, and he tries to convince everyone (including himself) that he's okay.  My basis for this?  He chose the lifestyle of a playboy actor.  A life full of phoney people and plenty of "masks" (roles) to hide behind.

So, there we go.  That's how I play Marco.  Now, as for when he's being pulled in from canon:  Post-Jake-ramming-the-other-ship.  Marco really thought he was going to die, and a deep dark part of him kind of wanted to.  And now he's here.  He's going to assume that the Ellimist is "fucking with him" some more, and will not acknowledge he was going to die, even though it was pretty obvious.

UPDATES:  Since arriving, Marco has discovered that he replaced an actor.  He's made a few friends, enjoys clubbing with beautiful women, and seems to generally be a pretty happy camper.

Then time hiccuped and his personal timeline was reshuffled.  Instead of coming here, he fell out of the vortex into the world of Fallout, and spent ten years surviving as a gun-for-hire in the Capital Wasteland.  There he developed impressive skill with various guns, and average skill with a sword.  He also learned that there are a lot more edible things than people think, and that you should only trust people as far as you need to.  During the week he spent as this other version of himself, he discovered that, had his timeline not been reshuffled, he'd have married one of those acquaintances he'd met, and apparently recently conceived a child with her.  He truly does not know how to handle this, especially not now that the timeline has been set right.  Though he does remember the altered timeline, he is actively covering it up.  He retains the skills he developed in the wasteland, however.

He also learned that his friends either already suspect his playboy lifestyle to be a front, or will soon.  He's unsure how to handle that, as he's not yet consciously aware of it himself.