Calm, cool, and a total cutie, Seiji is quite the character. Having a combination of coldness and sweetness, Seiji's personality is a complete enigma, since you never know what to expect next. His distant and cold attitude is only one-half of his many-sided personality. Courteous, stubborn, and patient, Seiji proves to be a lot to handle.
Patient as he may be, he's been known to blow a fuse a time or two. His lack of tolerance for Jun and several other members of the YST group, and his quick action to fight leave us with the fact that Seiji is quite a hot-head. Though planning and careful strategic discussion is always a must with Seiji, we all know that if provoked, he blows up just as much as any of the other Troopers.
Seiji's lack of emotion in situations leaves others to believe he's cold, and sometimes hateful towards people or peoples. He's very realistic and straight-foreward, giving you nothing but the hard truth. He isn't as cold as he makes out to be though. Though few and far between, he does have some 'special' moments that reveal how he really feels. We mostly see these moments when he's around Nasutei, or if something desperate is happening.
He's a rather outgoing person. He enjoys F1 Racing, Kendo (or anything to do with swords), and is skilled with the Bamboo Flute (and other Japanese instruments).
Seiji is the 'pretty boy' of the group. While he seems obsessed with himself in the American version by constantly referring to 'beauty sleep,' he doesn't mention himself much in the Japanese version. I hear, he's very humble about himself, but that too isn't completely official. ^^ I would think Seiji is rather vain, since he seems to wear slacks, suits, and other very nice clothing. In the series, there has never been an actual scene with Seiji wearing blue jeans.
Seiji, even though does seem quite calm and practical, does have his wreckless moments and does do some rather wild and foolish things. I like to look at Seiji as a split personality. Even though most of the time he lets his actions be controlled by his thoughts, there are occasions when his emotions take over. For instance, whenever he gets really upset, I mean enraged, he can lash out and fall into a blind rage even though most often he is seemingly mild-mannered. He also strives for adventure and action. He drives cars (illegaly) and even won a racing contest when he was 16. It brought quite an uproar when he drove Nasutei's car when he was 14. The legal driving age in Japan is 18. Even when we first get introduced to Seiji, we find him falling off a building head-first without messing up his hair.
Seiji's main dislikes are lies, evil, and talking with girls. One possible theory of Seiji's dislike towards speaking with women is due to his older sister, Yayoi. It is said that she has teased him so much, it's left him scarred, and unable to easily talk with women. However, this is unofficial information, since it does not state anywhere why Seiji has such a hard time speaking with women, although it would be the explanation that makes most sense.
Nothing is really official about Seiji having a best friend. While he seems most comfortable around Touma, nothing is stated about them being 'best.' YST Encyclopedia stated them as all being 'really good friends.' Shin and Shuu act as though they are best friends, but nothing is official about that either. Seiji does seems to have his clashing moments with Shuu, however, due to a strong difference in their personality.
As far as romance is concerned, a lot of it is left for the insane imagination of webmistresses such as myself. However, it's stated that Nasutei and Seiji may become more than friends later on in the series, since Seiji's really the only one who doesn't see Nasutei as a 'sister figure.'
Speaking of romance, Seiji apparently has a very old way of looking at relationships. According to 'Nanpa Call', Seiji said that he was too young to think about a girlfriend when he was possibly 16 (the ending scene of the second half TV series is Ryo's 16th birthday), so for Seiji, 16 is too young??? Go figure. That's to bad too, because if Seiji doesn't find a suitable woman soon, his parents might stick him into an arranged marriage. *screams* Oh no, isn't that terrible? Yes, but very possible...
I also wanted to clear another thing up in the 'World Of Romance.' If you've been keeping up with Ask Seiji, you might have noticed that there was a question that popped up once that asked 'Seiji, have you ever done 'it' with Nasutei?'; 'it' implying 'sex.' Well, of course, I said 'yes' for entertainment purposes. But if I were to be completely accurate about this, knowing how Seiji's personality is and his thoughts on relationships, he would most definately wait until after marriage to actually have sex with someone. Now if that's completely true, I don't know. Maybe he'd wait, maybe he wouldn't. It would be highly unlikely for him to get drunk and end up doing it by accident *laughs thinking about it*, so the chances of him doing it under normal circumstances are highly unlikely.