I have only drawn Nemo twice, both with very limited information, so I feel I need to rewrite his backstory.
Nemo, or Wallace, is a being from a place called the "Base realm", a universe contained within a forever expanding cube, populated by mathematical structures and geometric shapes, each with their own behavior and personality. They communicate via monotone sounds and ~MATH~, but these conversations are limited to greetings, when one being passes or bumps into another. They are all infinitely experts in math, being able to understand it in a way no human could ever imagine. One of these civilians however, a triangle with a hole in it, Nemo, somehow passed through the cube's wall and entered the human world. At first, Nemo was so overwhelmed by all of the sensations and the fracturing of his reality, he was an unstable shifting object, emitting a screech of pain and confusion so high it disrupted almost everything near it.
Eventually, Nemo calmed down, and attempted to observe human society, barely mimicking a human shape, adorn in any clothing he could find, mute and completely unaware of social norms. It took a long time before he found a way to communicate, via numbers, and he moved into an apartment with a human male. Though still extremely strange and touchy-feely with anything he got his hands on, he managed to be a semi-decent human imposter. As he continued life on earth, he found himself obsessed with the weirdness of humans and the things he never knew of, like food, sleep, entertainment, and friendship.
The longer Nemo stays on earth, the more "organic" his form becomes, eventually growing hair, skin, organs and vocal chords, but he does not look like a human, moreso a grotesque mangled body. When he first arrived, he looked somewhat like a mannequin, and his limbs often "glitched" and became much simpler geometric shapes.
Nemo enjoys reading (particularly science books), watching TV, eating any and all foods, observing human behavior, touching things and humans (if someone is not observant, he will curiously mess with their hair and face. Squish squish.), and math in all forms. Despite him being a rather passive thing, he is apathetic and has no real care for life if it has not benefited him, so he could easily kill if given the proper tools (he is not allowed within reach of knives and guns). He does miss the base realm at times, where his disappearance caused a panic, and his people actually began talking about it, instead of just saying "hello" to eah other.
His relationship with his roommate is somewhat unconventional. His roommate is an aggressive, inconsiderate and introverted man in his 30's with an office job, and he is very often annoyed with Nemo's obliviousness and whenever they're in public he claims that Nemo is mentally challenged in order to avoid embarrassment. Nemo likes him though, and his favorite activity is watching him sleep and breathe.