An accidental result of a drug addicted prostitute and one of her many clients, Eid spent his early years in and on the street around the brothel his mother worked and lived in. He was mute from birth as a side effect of his mother not letting up on her addiction during pregnancy. Luckily the other sex workers took a shining to him and often passed him around to help care for him (scrounging clothes from ragbins or charity shops, buying him treats if they'd had a good score that day etc) whenever his mother was too drugged up to do it/busy/didn't feel like it.
Later his mom would sell him to The Company in exchange for drug money. It seemed like a good deal.
Eid has spent at least one year in The Company at this stage, and always has at least one injury on him due to his training to become an assassin. They've started dressing him in darker clothes to get him used to disappearing into shadows/becoming generally unnoticeable in the dark, although his hair colour stands out a bit more than they would like (it does however, make it easier to distinguish him from most of the other students). He's been paired up with Mu as his long term team member and has gotten used to her talking his ear off.
Current Eid. No one knows where he got the mask from (it's possible he painted one he found during a craft session when they were supposed to be creating practice targets), and Eid wears it mainly so he's less likely to get the job as bait for some of their potential targets. Although he knows sign language this is essentially useless to him as most others he's in contact with don't, so he communicates mainly through writing implements and things such as Magnadoodles and Etch-A-Sketches. Mu is the only one that knows exactly what he's saying through observing his facial and body language cues; that's what being partners for six years will get you lol.
Eid considers Mu his BFF and tends to stick close around her. Unlike her, however, he's not overly concerned with longing for a normal family because he's figured out the chances of him having one is pretty negligible. He would rather much like it if he had more free time as opposed to going to "school" and "work" though.
Eid's in the midst of a growth spurt and has become somewhat lanky despite all the physical activities he does. His mask was an effective way to hide his face so when he outgrew his old mask he simply found and made a new one. The scarf also stays throughout the years as it is his preferred weapon - asphyxiation/neck breaking is far less messy to both the target and himself, although he does have a knife or two hidden in his jacket as back up.
Eid's known Mu is a girl for years, but it only really, seriously, clicks in his mind that AMG MU IS A GIRL at about this age. Darn puberty. How awkward.
Whilst Mu is infiltrating parties and the like out in the open, Eid is the secret one that helps out in the background. The Company need an almost literal "ghost" to eliminate someone for them, then they usually opt to send Eid. Security cameras interrupted or tampered with? Eid. Any accidental left over evidence from Mu's kill? Eid takes care of it. People in the way of Mu making a clean getaway? Eid will take them out of the equation.
Unknown to Mu, Eid also does this silently in the background whenever she sabotages The Company, checking up after the things she's done to make sure there are no discrepancies that can be tied back to his partner. He makes no mention to either party about what the other is doing.
Growing up has made Eid a little more shy about seeing his partner in too much of a state of undress unless absolutely necessary, just in case he reacts in a mortifying way that they would regret or would get in the way of their partnership. So you can imagine his surprise when Mu suddenly lays one on him one day and gives him a ten minute speech about the feelings she has for him, lol!