Basic & detailed profile information from official sources. Lots of spoilers.

official profile

This is Sakaki's profile as it appears on the official GH2 website.

I'm an adult, so I can do unfair and underhanded things.

The math teacher for ultimate level classes and the main character's homeroom teacher. As a special exception, he is also teaching the main character's regular level math class. He has a taskmaster teaching method, and when it comes to his classes, he has no use for students who are not devoted to learning, but those who take in his teaching make rapid progress in their learning. He leaves the student guidance to the assistant homeroom teacher, Ito Keita.

voice actor Okawa Toru (大川 透) ~ other roles
age 27
birthdate/sign January 12, 1990 (Capricorn)
blood type A
height 182cm (5ft 11.5in)
ability chalk throw (so says the site, but it's actually academics/math)
other parameters (scale of 0-5) luck 3 / intelligence 5 / popularity 2 / special ability 4 / physical strength 3 / influence 4

character data sheet

This is a direct translation of the character data sheet from the official GH2 data file CD, with a few extra notes and some minor editing for readability.

name age blood type birthday/sign
榊宗司朗 (Sakaki Sojiro) 27 A Jan 12 (Capricorn)
height birthplace personal pronoun often seen with
182cm (5ft 11.5in) Tokyo 俺 (ore) Sonoda (he makes him bar snacks)
club special skill specialty subject reason for being at BL
(blank) chalk throw math faculty member (and assistant director)
family configuration position affiliation
President of the Sakaki Group as father, mother, older brother (also in the Sakaki Group) math teacher, assistant director board of directors
interests underwear class sorting
making tests where it's hard to get 100 points (blank) teacher, specializing in ultimate level
favorite things favorite food dislikes
(blank) Japanese sake complaining despite not putting in any effort
Basically, he thinks that it would be best if he only taught students who are interested (those interested in learning/studying), and he's reluctant to teach the normal class, which has a lot of students who are simply in class to fulfill attendance requirements, including the athletes.
strong point weak point
(Although he wouldn't say it) He actually has a lot of faith in the students and is concerned about them. He only explains the bare minimum. If you don't understand, he thinks that you didn't put in enough effort.
special remarks
He's not in the flow of the main character's story, but you can go after him in a side route.

He is a math teacher, but is also the school's assistant director. Because he wields the authority of the board of directors, he has similar authority to the director. He was originally supposed to work for the Sakaki Group, but because a business partnership with the condition to cooperate in managing the school, he was allowed to be a teacher.
short history
This is his 3rd year at BL School (transferred at age 25). Before that, he was a teacher at a private school owned by the Sakaki Group, where he was famous for his disciplinarian teaching.
(Note: That private school was the high school Sakaki himself attended.)
hidden settings (revealed in the course of the story)
Assistant director. He presently wields the authority of the board of directors.

* The will of the board is to shut down the school, but because of Professor Sakaki's real feelings, he is not serious about shutting it down. He is just determining whether the school is truly fit to continue or not via the Bell One. Similarly, he's trying to ascertain whether Kasahara is fit to have the position of director. (If Kasahara is having a hard time having been made to be the director then he should quit, but if he voluntarily starts growing some determination, then he intends to support him.) He mobilizes Sonoda to stir things up in the school, but he is not considering helping Sonoda achieve his goal. (Because it's a problem Sonoda has to solve himself.)

Sakaki himself, because he was dispatched by his parents' company, the Sakaki Group, is executing the instructions from the Group (concerning shutting down the school), but he's just stubbornly persisting in putting forth a public face of fulfilling his duty (as long as it appears to the Sakaki Group that he's fulfilling his duty). Personally, despite any doubts he may have about the way BL School is, he doesn't think it needs to be shut down.
hidden settings (not made clear in the course of the story)
When he was a student, he hardly ever got any grades other than 100. (His fundamental ability to comprehend is high, coupled with the fact that he's proficient at deciding what is relevant or not in a piece of information meant he could learn very efficiently.) He is not really a genius, but he has no difficulty in making the effort to study.

A supporter of fiercely competitive entrance exams. He thinks high school students are too immature to judge what is necessary or not necessary. Because of that, he thinks it's good for them to study and work hard now, to increase their options for the future. (Or rather, for students who don't have some special talent, that's the only thing they can do for their future.)

He likes to make tests where it hard to get 100 points by mixing in questions that only high ranking students would be easily deceived by.
additional information
A smoker. Because all the buildings at BL School are non-smoking now, he smokes on the school roof.

extra notes, not in this profile but found in the game, etc
~ Sonoda is Sakaki's stepcousin. Three years before the events of the game, Sonoda's father disappeared, and his mother remarried Sakaki's uncle.
~ Received BL Platinum Paper (reason/skill unknown, but did not attend BL).
~ Sakaki's so-called "cool car" is actually a third generation (2010-2014ish) Toyota Prius! (source)
~ Conversations in the game suggest that he also teaches at least some of the hard level classes, but the details are unclear. He is also teaching Yuki's first year regular class.
~ His hair is naturally wavy and gets messed up by humidity in the rainy season. (source)
~ Has attempted to quit smoking, with Nao's support, without success. (source)