((Picture above drawn by WEH-KA))
Some years after the Chernobyl accident happened, Sergei was born. Little did he know he had lost a sister, Signe, when the accident occurred. Even in his time, the atmosphere continued to be packed with gases that proved to be lethal and deadly in various manners. Sadly, Sergei inhaled one of these gases, and as result it was shown to be poisonous to his lungs. Thankfully he was given a breathing mask to continue to live with the poison inside.
Sergei himself is a rather quiet individual. From a distance, he may look somewhat mean, but he comes off more as careless and stiff in reality. He prefers to stay in secluded areas, having social anxiety. He's shy in this way-- back in Chernobyl, he was often teased or made fun of because of the mask he wore, and that had an effect on him. However, on the inside, it has been shown that he's very attentive and thoroughly cares for everyone around him, especially his beloved sister, who is now raised from the dead as a zombie.
Because of his abusive and dark background, Sergei tries not to show much emotion. He sarcastically mocks the thought of having feelings, sometimes claiming he's not like others in experiencing certain parts of them. Having a mild type of depression, he's usually clouded in a grim mood, making him appear as so, but really he's like any other human being and knows this deep down, though often he beats himself up for it.
Sergei is also a gunmaker. Coming from Ukraine and having Russian history, he is very experienced in using weapons and particularly making them. He does commissions and makes money off of his work.