Shaggy blond hair that hits the nape of his neck, deep blue eyes, and an ever-slight slouch that gives off the perfect air of arrogance without losing that sense of laziness, this 5'10" young man is one of the most stereotypically contradicting people the world could meet. He's lanky with noodle-thin arms and a flat torso, unfashionable because he wears whatever the hell he feels like, and talks to his own shadow. Okay, that last one's pretty weird, but usually Pallas is hiding out of sight until needed. Regardless, he actually looks pretty good if he ever dresses up, but it's nearly impossible to get him to smile (and though his Pokémon has been able to, no one else has). He usually doesn't even frown! It's just a straight face and half-asleep eyes almost 24/7, but for some reason most of the girls at school go wild for it; something about the "I don't care-ness," they say. (They need to lay off the television.)
A double-sided glaive is Eren's only weapon. However, his mother blessed him with a great deal of endurance and stamina, as well as impressive strength whenever he's defending himself. (He actually doesn't like to start fights, if you'd believe it, but word that gets around claims otherwise. He'll even take a hit for Pallas if she's not doing too well.) Finally, while it may not necessarily be a power, he's inherently skilled with handicrafts.
There's really only one adjective necessary to describe Eren, but three will be used instead: Depressing, irritable, destructive, a little condescending... and bad with numbers. Due to such ease in describing his demeanor and cognitive skills, just about everything people know about him is learned from his actions. All others really said to describe Eren is that he's "like an emo kid and a dropout put together, but a thousand times worse, and without all the suicide, crappy generic poems or drugs."
His curiosity also ends up a major problem with him. He's rather intelligent in many subjects (except maths and his alternate language classes... and gym, though he definitely has the skill), but his desire for hands-on experience gets the best of him too many times. That said, he promptly forgot quite a bit of all that "math stuff" whenever summer started. It was nearly impossible to retain it compared to learning about a poisonous frog by dissecting it... and licking it... and going to the hospital. Thankfully, and apparently miraculously to the doctors, he recovered very quickly. Never did that again; got booted out of Biology. His school's staff was really only happy that they couldn't get sued for it. (Almost all the staff hated him, and on one account some even admitted it. Said he was a bad influence on the more dedicated students.)
On Eren's first visit to Camp Half-Blood, his attitude really stuck. However, there were people that tolerated him, even tried to make friends with him. By the end of summer, he was, of course still depressing, irritable and destructive, but he was much more aware of whom his actions affected. After two years at camp, while he retained his old demeanor, Eren's now much... friendlier, so to speak. (That is, he doesn't shoo someone away until after they've made their case now.)
Conceived with the goddess Athena in Idaho, Eren Ritter's biological father is named Sean Ritter. Only three years after his birth and Athena leaving him with Sean, Sean became abusive; unable to hold down a job and started drinking, but that's another story. Before any (physical) injuries were dealt, however, the goddess helped her child into a foster home, then an adoptive family in Seattle. (Sadly, his linked Pokémon, an Eevee at the time, wasn't able to follow him.) Frankly, that didn't help him grow up any better. He had an older step-brother, not a very nice guy until he'd entered high school, when he became sort of a role model (but not to Eren). Needless to say, Eren found himself enjoying the majority of his childhood playing around in the rain and eating Dick's hamburgers, alongside using violent video games as a stress reliever; these ended up being just some of the few things that made him happy enough to smile.
One of the other things turned out to be a Glaceon that'd wandered out of Alaska around the time he was in sixth grade. It looked happy to find Eren and stuck by his side, so the boy took it home and kept it. Even when his parents said no, it wouldn't go away, so they let it stay after just a few days; it's been with him ever since. It was around the time he entered seventh grade where his senses were beginning to get confused. He'd hear and see things, sometimes even feel things that didn't seem to be there. The problem faded within a couple months, but reemerged in high school. During ninth grade, Eren's looks alone were enough to get him into fights with the other students, the attitude was just the icing on the cake. Though he never did start them, he was always the one that walked away without injuries. As such, he was frequently in detention or getting suspended.
During one particular detention session, the "convicts" were assigned to a crafts room. Instead of just sitting at desks and sleeping, they had to clean up. Eren, rather than cleaning, found himself having picked up some tools and yarn and making a beanie for himself in five minutes flat. (Even had his first initial on the inside.) While it was definitely juicy info for the other students, they knew Eren's track record; as a result, his little-known skill stayed little-known. As a year of school passed, he turned out to be the only one left in the gym from a detention sentence. It was rare, but not unheard of for it to happen.
Then something even weirder than temporary schizophrenia happened. A disgusting, monstrous beast with a lion's body and a man's face burst through the doors, charging at Eren with a murderous roar. The first attack he was able to dive out of the way, while the beast slammed into the gymnasium's back wall. The second time he was even luckier. About to pull on a defensive stance to retaliate, a young woman stood between the two carrying a large shield roughly the size of a dinner table, as well as a nice-looking spear. A healthy glow on her skin, short black hair, and armor that would've fit a Trojan War reenactment, she was Athena. She turned to Eren and managed to tell him, "Hello, son, I'll explain in a moment," before practically just flicking the monster into yet another wall. The shield shrunk to a fraction of its former size, but that was probably just to give Athena her mobility back, because she immediately rained "fiery pissed off mother" on the monster, which she later clarified to be a manticore.
Soon enough, Eren was moved to Camp Half-Blood. Knowing he was going to get kicked out of his school eventually (and not wanting to deal with his step-brother), he very willingly stayed year-round with his Pokémon at his side. It was there he learned that the titans in Tartarus were growing in strength, and for the next two years he and Pallas trained for this purpose.