Some "world" info:

The year is 2057, a decade after a nuclear attack on the U.S. after subsequent battling elsewhere. Most of the earth is either dry or flooded from the affects of global warming. Many species of both plant and animal have gone extinct, and the human race is on the edge. In war-torn areas, people are just as sparse as the vegetation and wildlife. Cars are, for the most part, wrecked or inoperable, and solid structures and architecture have mostly fallen apart. Nature has tried to make a comeback in some of the least-populated areas, but it will take many more years before earth will even look a little like its old self.
The sparse communities are run by small committees, but most people just stick to their families, if they have any left. However, that doesn't apply to the mutated members of society, who have to hide in wrecked cities, caves, and underground. The mutants are deformed and often have scarring from the war and abuse from regular humans. Injured war veterans who aren't mutated will often be more accepting of mutants, though, because they themselves are often outcasts as well. If mutants don't travel in well-protected groups, they can be subject to attacks by groups of humans who have turned their fear into hatred. Mutants will often have more than the normal amount of some body parts, as well as disfigured faces and skin and can have certain illnesses that make it worse.

Trek, Muggy, and Artie stay just outside of a radioactive hot-spot within a dusty, old library full of many books that no one really cares about anymore. They often migrate from one area to the next, however, since Trek knows that even being on the edge of a radioactive area can be bad for the kids' health. Trek does his best to avoid areas he knows are populated with normal people, for the safety of the children, both physical and emotional, since mutants are called worse than just freaks and usually treated very differently, as though they were contagious.

Trek - 32
Before the nuclear attack, Trek (formerly known as Luther Whitmore) was just a regular adolescent. He had an abusive father and was drafted into the Long War before he had even reached his twenty-first birthday, but he was still relatively "normal." He participated in the war for many years, on and off the battlefield. During his years at war, he developed Post Traumatic Stress Disorder, as many who survived did.
He ran into some trouble with some younger soldiers on his side and was framed for killing a high-ranked official. He was going to get a death sentence, but instead, a scientist insisted on using him for an experiment instead of killing him. After that, he was taken away and his brain was removed and placed into an android container body.
The experiment was a success, but unlike others who later were tested, his new body was simply a skeletal system within a scarecrow-like covering (so they didn't waste money on a good body). Later "models" were filled with the brains of fatally-injured soldiers so they could still help, but their bodies were much nicer.
Not too long after he received his "punishment," the nuclear attack hit. He and some others who lacked real bodies were sent back to the homeland to check the damage. There were barely any survivors near the hardest-hit areas, but when he found the scientist's family, he decided to repay the favor of giving him immortal life and keep his pregnant wife and son out of harm's way. The other androids had more important things to do and didn't care if he was left behind.
With such radiation in the area, it didn't take long before the wife and son became gravely ill. The son died first, and after giving birth to triplets, the mother also passed away. One of the babies was stillborn and severely deformed, but the other two survived, leaving Trek to take care of them because the regular people were too terrified.
He's watched over the two ever since, having learned not to even notice their abnormalities and treat them as though they were his siblings. He's not heard from the scientist since he got back to the States, so he presumes he died.

Trek is terrified of planes, will often have nightmares, and can be startled easily by loud noises because of the war. Because of his new body, he's grown to like sewing and learned to avoid close contact with fire and too much rain. He also doesn't like to look at his reflection because it reminds him he's lost his humanity. The very core of his body has a copper, skeletal-like structure, but he still needs to keep the outside in tact to better protect his brain, encased in a brass skull.
He is rather intelligent and street-smart and also knows a handful of fighting and weapons usage techniques. When he's not stressed by the world around him, he can be rather easy-going, but most of the time, he's rather apathetic and humorless. Most of the joy has been sucked from him from the years of hardship he's endured. The kids can get on his nerves, but they also have made his heart a bit bigger. He's very protective of them, but tries to not show it.

Artie - 7
Having lived in a library for most of his life, Artie is rather smart and quite the bookworm. He adores classic literature and history, especially poems. When they can find it (which is rare), he loves sweets such as fruit and chocolate. He doesn't like swimming at all and would much prefer to be either on the ground or climbing tall things (whatever hasn't completely crumbled or fallen apart).
He can be rather quiet and reserved when Muggy's not around, but when they get together, he can cause some trouble. Usually, though, he's responsible and feels that he's Trek's "second" since he's a guy and is older than his sister (by ten minutes). His height bothers him a little bit, since he's shorter than his sister, but he swears that he'll be taller than both of them when he's older.

Muggy - 7
Muggy also likes books, but she enjoys science and even math sometimes. She'd rather pick up an educational book instead of something for pure entertainment. Her favorite food is eggs, and she's much more keen to eat vegetables than her brother. She doesn't mind the water as much as Artie and also enjoys climbing (whether it's Trek or to the top of a bridge doesn't make a difference).
Though she's very independent and free-spirited, she can be sensitive about her looks when around other mutants, and even more so with "regular" people. On her own, she'll be all over the place, exploring and examining things, but with Artie, she often is prone to doing less thinking and instead bugs him constantly.

Muggy and Artie, shared traits
They were born deformed for the main reason of their mother being exposed to radioactivity, thus causing them to mutate whilst in her womb. Trek has never treated them any differently than other kids, so the fact that they're deformed doesn't phase them too much (or at least when they're alone). Despite that they never knew their parents, Muggy and Artie never really saw Trek as a father to them; he's always been their "big brother." They're one another's best friend because they don't interact with other kids very well (on the rare occasion they find anyone around their age), but that doesn't bother them; they're used to it.
Though things they do often annoy Trek, he doesn't lash out at them (at least not often), but when he seems upset, they usually get the hint and leave him be. One of the things they do that annoys him a bit is completing each others' sentences. They're not extremely great at it (since Muggy tends to change what she was going to say to something less expected), but it's a good game for when they're not doing anything important. They're pretty much little monkeys with the way they get into things, climb, and cause trouble, and to those watching that don't have to take care of them all the time, they can be pretty amusing. It's just a good thing that Trek's pretty patient or they'd have been dumped long ago.
The two have a slight brain disfunction that makes them lose control of their motor skills temporarily so they either make erratic motions or cannot move at all. These occurrences can sometimes cause problems, and though they don't happen often, Trek will have to carry them to a safe place to wait the time necessary (usually ten minutes to half an hour).