What is Echo?
Echo is something called a simulacrum. A simulacrum is basically a clone of someone from another world. The world Echo comes from has a weak barrier between itself and other worlds, which allows for dimensional information to slip in that shouldn't... and sometimes, when information on a person slips in, a copy of sorts is created. That copy would be a simulacrum. (So there's no crazy bio-processes involved. It's all dimensional shenanigans.)
Simulacrums typically have laser-guided amnesia. They don't remember their names or the lives of the people they were copied from, but they remember things like how to talk and do basic functions and whatnot (rarely, they may also remember special skills that their original possessed). Since they don't remember their names, they tend to just pick something that they think suits them or have one given to them by someone else (this results in a lot of people with weird object or color names).
Simulacrums can regain memories of their originals over time, but the interference from dimensional energy will make them sick. Typical symptoms include nausea, vomiting, fever, achiness, other assorted flu-like symptoms, and psychological symptoms like anxiety. Typically, a simulacrum can be sick for roughly a few days before regaining a memory. Echo is a little special in that he has a higher level of dimensional interference than other simulacrums, which means that he gets much sicker for longer. While most simulacrums can typically push past their symptoms, Echo's spells tend to lead to him flat-out collapsing in the middle of where he is and being more or less off his feet for roughly a week, if not more. (As a side note: a simulacrum can get sick from dimensional interference and not regain a memory.)
Something in a simulacrum's composition allows their souls to be downloaded out of their bodies and used in something called the Simulacrum System, where their souls can be summoned and used to fight nasties that slip in through the weak dimensional barrier. The souls can be put back, mind you (the bodies are put into stasis while the souls are being used), but this is every bit as morally grey as it looks. Echo and his friend Gold are the ones who lead a rebellion against the organization using the Simulacrum System, in fact.
Personality:
To use a few words, Echo is bright, energetic, and passionate. He has a certain special zeal for life and all its experiences, which tends to lead to him flying into new things without thinking. He is very emotionally warm and friendly, which has put him on good terms with just about everyone in his little village of simulacrums. If you become his friend, expect him to become incredibly devoted to you (perhaps a bit overbearingly so, given the intensity of emotions he can feel and just how he wants to express it).
Echo is something of a sensualist. He believes that every simulacrum should try to make new memories to replace the ones their bodies keep from them, and this includes sensual memories. He loves food (to the point of being something of a glutton), comfy surfaces, physical intimacy (this is a big one for him; sometimes he doesn't seem to understand why he can't just express his feelings through gratuitous hugs to everyone around him)... those kinds of things. If he's fond of you, he will get touchy-feely unless you tell him off, in which case he'll respect your wishes but be seriously bummed about the whole affair.
Echo draws inspiration from what memories he has of his original. He thinks that his original was some kind of great leader or hero, or at the very least he had a lot of friends. To "make Dad proud," he decides that he wants to draw people together for some great cause, that cause being protesting the use of the Simulacrum System and perhaps doing a little damage to the organization responsible for it.
History/Relationships:
About a year prior to the story, Echo was discovered outside a village of simulacrums. As per procedure, he was given a rundown on just what he was, what was going on, so on and so forth. When he was asked to pick a name for himself, he chose Echo because "Aren't we all just echoes of another person?" His stance on this would change quickly. About a week or so after his appearance, he was brought to the facility behind the Simulacrum System for registration in the Simulacrum Index. Here his higher level of dimensional interference was discovered, and he was warned about what this meant for him.
A month after Echo's appearance, a silver-haired simulacrum was found by the facility. The scientists at the facility discovered that this new simulacrum came from the same world as Echo did, and so they contacted Echo about meeting him. Echo was the one who first met Gold and gave him his name (for the gold accents on the jacket he arrived in). The two became fast friends despite Gold's fearful and distrusting tendencies (one can't blame him for having trust issues when the scientists found him first and did research on him). Echo's sheer emotional warmth seemed to comfort Gold, along with the fact that they shared an original world.
As time went on, their bonds deepened significantly. Somewhere along the line (probably thanks to Echo's intimacy-pushing ways, both emotional and physical), their relationship became so intense that they could basically just be called lovers ("really, really intimate best friends" doesn't seem to cut it). The two bonded over shared experiences and the trials of being a simulacrum. Together, they will lead the rebellion against the Simulacrum System.