Here's what we know about Marty: he's charming and "good at like anything" (2.6). And… that's about it. He's a flatter-than-Flat-Stanley/ flat character, mostly because all he wants in the whole, wide world is to be Link. For example, after Link tries to "skeeze on Violet by showing off his lesion, Marty tries to get in on that action:
Marty was trying to make up lost ground by saying, "Maybe you could change the bandages on my foot," but that was clearly just disgusting to everyone. We were all like, "Unit, no one wants to see your damn foot," and, "Jesus, Marty unit, stow the mess-hole." (4.35)
Poor Marty. He may be hot (and not resemble a dead president), but he still comes off like a "broken little economy model" (4.39) of Link.
Sometimes, characters are flat because the writer doesn't have the skill to make them round. Sometimes, characters are flat because the writer just isn't interested in anyone but the main character. And sometimes, the flatness is the point.
Check it out: we don't even get any kind of a voice from Marty, so there's really no character development to speak of. Most of what he says is fairly incomprehensible, all "You broke off a... a thing […] You broke off a fuckin' thing" (5.20). Uh, thanks for the clarity.
But then, M. T. Anderson literally takes away his voice:
Marty had also gotten a Nike speech tattoo, which was pretty brag. It meant that every sentence, he automatically said "Nike." He paid a lot for it. It was hilarious, because you could hardly understand what he said anymore. It was just, "This fuckin' shit Nike, fuckin' you know, Nike," etc. (55.3)
Get it? By the end of the novel, corporations have literally taken over Marty's brain—like an amped-up version of a kid going around wearing Nike sneakers. After all, if you think about it the right way, wearing a brand's logo is essentially paying them to advertise.