Tom Tanaka (田中 トム, Tanaka Tomu) is an old friend and senpai of Shizuo Heiwajima. He works as a debt collector and is Shizuo's employer.
Tom is usually seen walking around in a suit, but his striped, unbuttoned shirt and his dyed, dreadlocked hair prevent him from looking 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. He does not work for a bank, instead owning his own debt collection agency. The light novels state that he collects for an online dating website.
He generally has a reserved, laid back personality, and as harsh as one might expect an underground debt collector to be, he is usually calm, even when confronted with Shizuo's destructive rampages. He is one of the few able to calm Shizuo down and make him stop fighting without angering him and incurring his wrath.
Tom and Shizuo met when Shizuo was entering middle school. Dengeki Magazine included an omake story that expanded on their middle school history. Some of the older kids tried to pick on Shizuo, but were soon beaten up; Tom approached with the following goals: a.) learn how not to piss Shizuo off (and thus graduate in one piece) and, b.) enlist his help in moving the injured seniors to the hospital. Tom bought Shizuo dinner and advised him to dye his hair blond so to help cultivate a reputation and fitting appearance that would keep punks away. Later, the senior delinquents praised Tom for taming "the rabid dog" and asked him to enlist Shizuo as a combatant for them. Tom refused, reprimanding them for calling Shizuo a dog and for disregarding his hatred of violence. After Tom left, the seniors planned to attack him to trap Shizuo; unfortunately for them, Shizuo had been drowsing nearby for the entirety of the conversation and at that point attacked. The seniors do finally corner Tom, who held his own just barely until Shizuo arrived and took over. When asked by Shizuo why Tom, who can't fight well, didn't just use Shizuo, Tom answered that although he's not good at fighting and Shizuo is, Shizuo hates violence, so he should not fight.
After that, Shizuo dyed his hair and enjoyed a peaceful period as his reputation kept triflers at bay. Tom and Shizuo attended different high schools (and Izaya Orihara undid all of Tom's work). Much later, when both of them were adults, Shizuo was having trouble keeping a job. Tom met Shizuo on the street and decided to hire Shizuo as a bodyguard.
Thanks to the fact that Tom is able to control Shizuo, and Shizuo's strength and destructive tendencies are actually a good quality to have in his line of work (instead of problematic like they were for his previous employers), Tom became Shizuo's permanent employer. Shizuo's job is to "basically beat up people who can't pay up," and generally to stay around Tom, protecting him. Tom has also made it clear that Shizuo doesn't need to beat everybody up, the idea being to use Shizuo's name and reputation to scare people into paying their debts. Shizuo, unfortunately, does so anyway.