personality...
Seiji, though polite, is hardly "soft-spoken." There is nothing "sweet" or "sugar-coated" in his mannerisms; rather, his emotions are controlled to a point of necessity, meaning circumstances dictate how he responds/behaves in any given situation. He is polite and formal in social atmospheres, though in battle, he retains a realism and practicality that is often mistook for heartlessness, cruelty, and ruthlessness. Seiji is a creature of logic, emphasized through his standings on involving civilians in warfare (i.e. "I just hope Jun and Nasuti don't get in our way." - Episode 12), and his sharp reprimands for carelessness in battle (i.e. "Bravado like that will get you killed." - Episode 9).
On the other hand, Seiji is young, short-tempered, and aggressive. Though cautious and prepared, Seiji often takes initiative in battle, and is easily roused into disagreements with other Troopers. He is often at odds with Shuu, the other aggressor of the party, and is not above telling someone they are "in his way" (i.e. "It would be faster to search from scratch than have you around." - Episode 7). What Seiji "thinks" often contradicts how he "behaves."
Once again, Seiji is "polite," albeit not above causing mischief (especially towards those he finds impolite/rude). In Tenkuden, Nasuti prepares lunch for the Troopers, though Touma, who is reading alone in a tree, decides to eat later, at his discretion. Seiji finds this inconsiderate, and sets fire to the base of the tree, saying, "This is useful when you want the bees to come out from their nest." Likewise, Seiji is, at times, deliberately rebellious, (for example—driving illegally), despite his upbringing.
Seiji is, however, understanding, lending comfort to Ryo during Episode 12 (i.e. Ryo slaps Jun for his own good), and sympathizing with Suzunagi's feelings in Message by relinquishing his yoroi (i.e. he holds her as she cries). And he has, more than once, shown an affinity for nature.
Though outspoken in his opinions, Seiji is typically mild-mannered and reserved. He is easily the quietest Trooper (or one of), often fighting alone and separate from the group. He is, in every respect, a loner (i.e. "You are an unsociable person since old times." - Youko, Korinden).
relations...
Seiji and the Troopers are "really good friends," as stated in YST Encyclopedia. He has the "tendency" to clash, especially with Shuu, and on more than one occasion with Nasuti. I would assume, however, he is highly respected, especially by Shuu's comment in Korinden (i.e. "I'll be beaten into a shit by the others if anything happens to you here!"). At the household (i.e. Nasuti's mansion), however, Seiji rarely argues (or rather, "goes with the flow"), as Touma says in Tenkuden, "Seiji's properness in following all the rules has no one who could match with him."
During Kikoutei, Seiji and Ryo are left to fend for themselves in the heart of Africa. Ryo looks to Seiji for advice, who more or less "takes charge" of the situation (which is odd, given Ryo is the leader, unofficially). It's probably nothing, though it gave me the impression of Seiji as a "right-hand man" of sorts, or at least a respectable/admirable figure.
Seiji and Touma are not "best friends," though their interactions lead you to believe so. YST Encyclopedia says the Troopers are "really good friends," and anything beyond that is simply fan misconception.
The most popular (and widely accepted) heterosexual pairing for YST is SeijixNasuti. Though the series has NO official pairing—and being a shounen anime, romantic ties are not important—the SeijixNasuti coupling is the most plausible, and hinted at throughout the series. SeijixNasuti doujinshi is very popular, and official images/scenes give way to an underlying relationship between the two. SeijixNasuti is odd for two reasons—Seiji is not the star/lead Trooper (though he certainly seems favored by the production team), and Nasuti is three years his senior, ages fourteen and seventeen respectively when the series begins.
On a side note, Seiji's household consists (almost) entirely of women (save himself, his grandfather, and his father). Women are more powerful in Seiji's family (i.e. "Seiji must go through a lot, in a family where women are more powerful!" - Shuu, Tenkuden).
official...
Seiji is classified as the series' "pretty boy." He dresses nicely, usually in slacks and dress shirts. There is no scene throughout the series where Seiji is dressed in blue jeans.
In seventh grade, he was vice-president of the Kendo club. By eight grade, he was president.
Seiji drives cars—illegally—and won a racing contest when he was 16. He takes Nasuti's jeep for a spin in Episode 20 (i.e. when he was 14), much to the surprise of Nasuti and his fellow Troopers (viewers were shocked as well). The legal driving age in Japan is 18.
Seiji dislikes lies, evil, and speaking with women, though why specifically is anyone's guess. Yayoi (i.e. his elder sister) is often given blame for Seiji's "problem," though YST Encyclopedia says only that she "influenced him," whatever that means (i.e. it's a fairly ambiguous statement). This applies to "strange" women, I assume, as Seiji is perfectly comfortable with Nasuti.
Speaking along the lines of romance, Seiji apparently has an old (i.e. traditional) way of looking at relationships. According to "Nanpa Call," Seiji said he was too young to think about a girlfriend when he was possibly 16 (i.e. the ending scene of the second half TV series is Ryo's 16th birthday), so for Seiji, 16 is too young? It's "possible" Seiji's parents may force him into an arranged marriage, were he unable to find a suitable woman (which remains speculation).
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