These are the official statistics for Tohma from the manga - any obvious discrepancies with the anime are noted.
BIRTHDATE: November 20th Star sign notes
BLOOD TYPE: B NOTE: Not a type A?! I'm shocked. Every other character I like is type A... blood type notes
HEIGHT: 175cm (5' 9'') NOTE: Bloody hell, he certainly never seemed that tall on the show! Just goes to show how much time he really does spend sitting behind that desk.
WEIGHT: 55kg (121lbs)
HAIR COLOUR: Blonde (white-blonde in the anime)
EYE COLOUR: Black (turquoise-blue in the anime)
Totally biased observation: he's a lot prettier in the anime than in the manga! Mind you, that's true of virtually all the characters, in my not-so-humble opinion.
character, background and speculation
At first sight - driven, ruthless, efficient, not at all bothered by running roughshod over other people's feelings if it means getting the right result. Eiri says he 'gets rid of people who are worthless right away. He doesn't just let people fade away.' He forces Bad Luck into having a new member when they don't want one - because he knows it'll make them a better band. He sacks Sakano as manager - because he knows Sakano's too nice, too neurotic and too disorganised to be a good manager, and makes a far better producer. He never does anything for no reason, and always seems to be calculating for the best possible results. That's not to say he's needlessly cruel - he just doesn't let consideration for other people really affect him too much. The same ruthlessness seems to extend to his own emotions too, as he's willing to let Eiri see Shuichi even though he is himself in love with Eiri, because he knows that Eiri needs someone uncomplicated, and because he thinks the publicity would be good for Shuichi's career. His own emotions don't enter into the matter - as long as Eiri's happy, he'll willingly put up with the situation.
Looking a little closer at the character, we can see a slightly different side to his personality - in relation to Eiri. Even Mika doesn't command this level of devotion from him. Even though he's not openly demonstrative, it's clear he cares very deeply for Eiri - he's willing to put an end to ASK purely because of the effect Aizawa is having on Eiri. The way he tells Eiri it's difficult for him to talk about the relationship with Shuichi is one of the sweetest things in the series, especially coming as it does after the scene in which he's cooking dinner for them. Terribly, terribly sweet, and that's not a phrase you'd usual associate with Tohma. This man has layers.
Like the vast majority of characters in the anime, with the exception of Eiri and to a lesser extent Shuichi, we know virtually nothing of Tohma's background and upbringing. In other words, this section is almost totally based on my opinions, though I've tried to base them as far as possible from proof in the show.
Tohma doesn't appear to have any siblings, and I'd say that was fairly consistent with his behaviour. I wouldn't say he exactly has classic only-child syndrome, but his constant desire for perfection both from himself and those around him would seem to argue quite a large amount of parental pressure in his past, which is often more the case with only children. With regard to the subject of parental pressure - I'd like to put forward a case for the Seguchi family being a little bit crazed. Both Tohma and Fujisaki, his cousin, display an occasionally terrifying thirst to succeed and prove themselves. Fujisaki is only sixteen, but is already by far the most professional and serious of the members of Bad Luck. Speaking personally, this suggests to me that perhaps someone like a grandparent of both of them was a musician but never a successful one, and rather lived his dreams through his children, forcing them into music. Tohma's parent, as the older sibling by some year's to Fujisaki's parent, would have rather more pressure, and this thirst for success was transferred on to the next generation - which explains why Tohma is perhaps even more frighteningly ambitious than Fujisaki. It seems to be rather the case that, in the clan Seguchi, you're a failure unless you've had your first platinum album by the age of twenty.
The same slightly strained familial situation could help explain why Tohma is in many ways such a cold and unemotional character. Despite his ridiculously cute and youthful looks and his frequent sweet smiles, he seems to have an ability to cut himself off almost entirely from his emotions. The episode where he pushed Aizawa of ASK in front of a car is a case in point - admittedly he did it because he was trying to protect Eiri, but a man who can threaten somebody's life with a pleasant smile is not someone who is entirely well-balanced. His extreme coldness to Shuichi towards the end of the series can be viewed in the same light. He doesn't seem to have many close friends, and seems to avoid showing any weakness in front of anyone - after Eiri's collapse in episode 11, he walks away from Ryuichi, Noriko and K - arguably three of the people he should be most comfortable with - because he doesn't want them to see him grieving; indeed, Ryuichi seems to be the only one who even realises that Tohma is upset.
So, Tohma is ruthless, brilliant, a master at suppressing his own feelings for the greater good, but still capable of great devotion, even to the point of obsession. You get the impression that he's adored Eiri practically for ever - when he upped sticks and went with Eiri to New York six years ago, that must have been when Nittle Grasper was either on the way up or still huge, and giving up all that for the sake of a friend's happiness is absolutely amazing. Eiri has *always* been the most important thing to him, which really makes the end of the series when Eiri makes it clear he doesn't need Tohma any more all the more heart-rending. Glad as I am for Eiri and Shuichi, Tohma must have been completely screwed up by this particular happy ending. What he'd do after such a rejection is - at least in the anime - left up to the viewer's imagination.