Seiji's probably one of the most mixed up characters in YST. Several different versions of him have been portrayed across the internet. Many myths have been made up about him due to a lack of informational recourses for the series. Fortunately, you have I, the all-knowing *snorts*, here to clear things up.
+ Seiji is a flirt. +
Probably the phrase that annoys me the most. Seiji is NOT a flirt. I'm not really sure when this myth was made up, but it seems to hold firm throughout many YST websites. Sometimes people separate 'Seiji' and 'Sage,' making Sage the flirt and Seiji the one who dislikes talking to women. However, 'Sage' and 'Seiji' are still the same person, just different names for the sake of dubbing. I focus more on the 'YST' version (Seiji) then the 'Ronin Warriors' version (Sage) simply because I don't always agree with dubbing. Anyway, the fact is, Seiji isn't a flirt, case closed.
+ Seiji is half-American since his father/mother is American, thus the reason he has blonde hair. +
Wrong!!! Seiji IS NOT half-American; he is fully Japanese. If you look at his childhood and family life, you'll note that Seiji's father married into the Date family, meaning he took the wife's last name instead of the wife taking the man's last name. The Date's are a respected, traditional family. Therefore, allowing a foreigner to marry into the family would be STRICKLY FORBIDDEN. Also, Seiji's father is a police officer, and there is no way they would let a foreigner join the police force in Japan. And I don't see how the mother could be American when she's the daughter of the man who owns the dojo. ???? As far as Seiji having blonde hair, I think we all know how Anime often times gives Anime characters funny hair colors. Usagi has had blonde hair since Bishoujo Senshi Sailormoon began, and no one questions her ethnicity.
+ Seiji is afraid of women. +
Um, no. This is a mis-understanding of words. Seiji 'dislikes' talking to women, he is not afraid of women. There is a strong difference between the two. 'Dislike' is not wanting to, or vexed to do so. 'Afraid' would be scared and running away from the women, and Seiji does not do that. He simply finds it difficult to speak with them since they flirt with him and throw themselves at him constantly. I'd like to note that it is implied that Yayoi is the reason for Seiji's dislike of speaking with women. She has teased him so much, that he is easily vexed around women. (Also note that Seiji dislikes 'speaking' with women, he doesn't 'dislike women.')
+ Seiji is 'King Flirt' of Hana High School. +
First of all, I think we established that Seiji is not a flirt. End of story. Second of all, Hana High School doesn't exist. That is a school that was simply made up in Fanfictions and has gained a reputation as being the Troopers' school. For one thing, the Troopers would never attend the same high school, unless attended while at Nasutei's mansion, for the boys live too far away from each other. Their homes are located in different areas of Japan, so therefore they all go to different schools. Touma and Shin are childhood friends, so perhaps they attend the same school, but that's it.
+ Seiji and Touma are best friends. +
Wrong again people. It is never officially stated as the two of them being best friends. According to YST Encyclopedia, the Troopers are 'all good friends.' Each is a friend of the other, but there is nothing stating any of them as being 'best friends.' It is simply a similarity in personalities that people come to the conclusion that Touma and Seiji are best friends. That and the fact that if Seiji is ever seen with a Trooper, it is usually Touma.
+ Nasutei and Seiji are a 'thing.' +
This is also not official. Any pairings made up with Nasutei are simply fanfiction. There is no actual information stating that she gets together with any particular Trooper. However, most evidence is pointed towards Seiji, and it is even hinted they may become something more later on. Seiji is also the only one who does not see Nasutei in a maternal/sisterly way, therefore adding more to the possibilities. I myself have an entire section for Nasutei and Seiji because I believe that they really would hook up, and that they are the only couple that even comes close to making sense, or has any hard evidence.
+ Seiji's a heartless bastard who is cold towards his friends. +
Seiji is not a bastard. Someone who who has the heart of 'REI,' or 'courtesy,' I highly doubt is a bastard. Seiji shows less emotion then the other Troopers, making him seem unsympathetic and uncaring. Seiji is the most aloof character in the series, but he isn't a bastard. There are a few moments when Seiji portrays an emotion of understanding, sympathy, and caring. However, Seiji does have problems with interpersonal relationships, making it difficult for him to always get across what he's feeling. This is one reason he couldn't save Suzunagi in 'Message.' I won't go into detail about the whole saga, but Seiji was unable to save Suzunagi by himself, he could only open a pathway of kindness in her heart for someone to come in and teach her what she was doing was wrong, or something to that nature. (She blamed the Troopers for the death of her parents and was capturing them each through their yoroi. She captures all except Seiji, who sympathizes with her and willingly gives up his yoroi without question, and Ryo.) Seiji didn't understand human emotion enough to save her, but Ryo did, so Ryo was able to save her spirit. If you want a summary of 'Message,' you can always check out Theria's Yoroiden Temple. (Seiji's problems with interpersonal relationships is the thing I love best about him. ^^)