Tom Tanaka is an old friend and senpai of Shizuo Heiwajima. He works as a debt collector, and later becomes Shizuo's employer.

Tom is usually seen walking around in a suit, but his clothing and his hair don't make him look too formal. This is probably due to his job as a debt collector, which needs him to look sharp and business-like but not too polite and friendly, as he often confronts people. It is likely that he works as an underground debt collector, and not for something like a bank. The light novels state that he works for an online dating website.

He generally has a sort of reserved, laid back personality, and as harsh as you might expect an underground debt collector to be. He is usually calm, and is able to calm Shizuo down and make him stop without angering him and incurring his wrath.

Source: Durarara wiki