Master Chief. Now, before everyone starts flaming me for that, hear me out. True, his backstory isn't explored a massive amount in the games, but the books and comics do give a big insight, and in Halo 4 a large part of the backstory comes into play. There are also plenty of references in Halo 4 to the backstory of all the Spartan-IIs as well. SPOILERS BE HERE FOR THOSE WHO HAVEN'T PLAYED IT, ANYONE WHO DOES CARE WILL ALREADY KNOW THEM THOUGH, BY THIS POINT...
Kidnapped aged six, forced against his will into an intense training program to turn him into what effectively amounts to a child soldier. Fighting against other humans, by the way (this was long before the Covenant turned up), he witnesses half his adopted 'family' (i.e. fellow kidnapped child soldiers) killed in horrific surgeries that would make even Dr Steinman think twice (well, maybe not) for the purpose of making the surviving kids even more powerful as soldiers. Also, bear in mind the entire project is run by the government's military arm, and overseen by a young (at this point only in her mid twenties) woman who has a reputation for being distant and mildly sociopathic, as well as being an obsessive control freak. Of the survivors, about half disappear, spirited away by the intelligence services on black ops, over the next 30 years while almost all of the others die in various missions - about 90% of those survivors die on the guys last op before the games begin, meaning as far as Master Chief knows he's the only survivor of all those kids he was raised with and trained to fight with. Oh, and his very first mission where he killed fellow human beings? One death on his team, where he was team leader. And he was only 15 years old.
Also, he has a childhood friend who he meets while fighting in New Mombasa - he can't tell her it's him because the government/military won't let him. Not to mention that although he finds he isn't the last one of his kind left after all, those who are still around are seperated from him again on further ops, and another half of them die just after he finds they're still alive. The next time he meets the woman who raised and trained him and his fellows, she's in custody for war crimes and kidnapping one of his comrades. And finally, the one constant for the past 5 years of all this, starts to slowly go more and more insane and finally 'dies' in a self-sacrifice which still fails to stop an entire city being massacred.
Yeah, given the sheer amount of pent-up PTSD here, I think John-117 is pretty damn tragic.
