>Does he have any gap moe traits or hobbies?
Yeah he seems like an evil, intimidating, and maybe kind of dumb henchman at first. In fact, he really plays it up too, but this facade starts to fall apart after some time. He was experimented on and treated like an object for 10 years, losing out on a decade of young adulthood. He's understandably very very angry but in his heart he just wants his pain to be acknowledged, for something to come of it (get ajin rights). I think this is why he so affected by Naomi Li's apology.
>What's his place in his source material, how does he effect the plot and the other characters?
He's Satou's, the main antagonist, sidekick; he became that way when Satou saved him from years of being experimented on. Eventually, he and Satou decide to take revenge on the people who imprisoned him, only Satou is in on it because he's bored. Tanaka takes it very seriously. The whole story kinda starts as a result of the consequences of what happened to him.
>Does he have any close friends? If he does, what is their friendship like and how do they treat eachother?
Kinda but not really. Eventually Satou recruits a group and Tanaka interacts with them which is kinda fun because Tanaka is so serious and the other members are way more laid back and like to get high. For a long time he and satou were THE pair and a lot of fun to watch but Tanaka changed a lot after Naomi's apology