Trevor is a very simple concept which is basically an exaggeration of the average going psycho GTA player. I wouldn't go as far as to say he's plastic unlike many 'popular' heroes he does have some depth and background to him but he is far from deep. Especially compared to Johnny.
Trevor's entire design was based around the 'Hey I'm random'/Exaggeration of a GTA player concept. Deepness was never going to be a draw for that character.
Then I don't think we should have been bullsh*tted into thinking he WAS.
He had such a deep involvement in Michael's story, in which he has one personallity of the violitile but not like "clinically insane" individual.
Side missions and Rampages say something different. There's missions like Nigel's where he sometimes takes the moral high ground, and is really apathetic. Or, there's rampages where he just kills everyone from being minorly pissed off.
R* didn't establish a consistant identity for Trevor, or even for Franklin; the guy who seemed to care for his fellow man and would only take professional jobs, but would do any two-bit, crappy gangbanger type job that came across.
Michael seemed consistant, but this is because he didn't have to change his personallity for all the Strangers and Freaks missions (BECAUSE HE HAD LIKE TWO THE ENTIRE GAME.)
Rockstar was too ambitious, they failed in intigrating Trevor sufficiently and consistantly into the main story because he was designed for the lunacy of side-missions and free roam. Hence, why he is given the special ability of proficient piloting from the start, all the real odd clothes, and perhaps the most powerful and least realistic special ability.
Franklin is like the inverse, but to a much lesser extent. I doubt the straight-edge young guy would give a f*ck about towing cars for some deabeat like five times after saying it was his last after the first.
Michael was more reminiscent of someone like Niko.
His character was carefully designed to fit over a well-written story; consequently there were no specific side missions for him beyond the world activities.
You can have a stupidly fun and funnily stupid gameplay experience, like Saint's Row, probably the reason they have you play as a custom character with no name or identity.
Or, you can play an extremely well-tailored story with an interesting and likeable character, like Vito in Mafia II.
My exigence for this, is that R* tried to do both with Trevor. You cannot make a character like him who's properly involved in the story... or at least they missed the mark.
Sadly for us, there is a greater portion of the gaming and GTA audience that gives a f*ck about planes and killing hookers than realism and a good story. Hence, why valid CJ in brown and ugly San Andreas got more love than Vercetti in glamorous Vice City. Why Luis with an attack chopper and an AA-12 got more attention than the deep and morally driven Johnny.
This isn't WRONG for players to favor this, it's just not MY preference.
A game I played recently really put this into perspective; Hotline Miami. The story, essentially, is an anti-story. Without giving a lot away (go play it, it was awesome) its story is deliberately flawed and bland to tell you at the end that if you were expecting a story from a game with already good mechanics, than you're not really doing it right.
While I ULTIMATELY disagree with this thesis of theirs, it was a very interesting and new perspective, and I know that the story driven game is becoming SUCH a niche genre; hence why games like The Last of Us are so prided for their stories. I didn't think it was anything special, but there's less competition for that genre now.