In the replies someone raised the question of whether or not Raizou calling herself a girl is consistent between the fan scans and the Japanese, so I'm going to go through and compile a non-exhaustive list of instances Raizou is referred to as a woman here:
In her first appearance she refers to her ambition of becoming the world's most beautiful 美女 (beautiful woman) two times in a row. She's also fine with other feminine terms such as 女帝 (empress), of course.
Further, she refers to "boys" as a group she excludes herself from:
By comparison, the evidence for her being "merely a feminine gay boy" is low to non-existent. To begin with, she expresses no attraction to boys through the entirety of the manga - the only times she swoons over guys at all are confined to the gag manga section Tsunekawa's Daily Life, but lest you think that one makes her less trans...
Her column in it is called "Raizou's Girl Talk" (乙女通信):
In her strip in the same section, she complains to her father that she was giving a masculine name instead of a feminine one like Momoko or Seiko, and when her father says that such names aren't for boys, she immediately tries to excuse herself from the situation:
Resulting in her father getting physically violent at her body language being too feminine. Raizou's transness is perceived and punished by those around her pretty consistently (see also: Jacob fawning over Jaibo 'if he was a girl' but calling Raizou categorically gross).
Her other column "Raizou's diary" features smash dysphoria hits such as: 「男も女もお尻は丸い」 by 雷蔵
だから雷ちゃんは鏡でお尻を見るのが好だって･･･女の子だと思えるから･･･クき! スン
"Both men and women have round butts" by Raizou
That's why I, Rai-chan, love looking at my butt in the mirror... Because it makes me feel like a girl... sniff.
Raizou is also not the only trans girl character Furuya has drawn. In Genkaku Picasso (2008 to 2010), he features Jeanne, who is even more explicitly a woman:
Jeanne hits a lot of the same beats Raizou does, though not living in an 80s helltown in the eroguro genre, she gets a happy ending.
Genkaku Picasso came out between the LHC main manga and the prequel, so it's pretty much impossible to argue Furuya didn't know what he was doing when he added those same beats to Raizou as well.
And all of this just covers the main manga and not marketing material such as Raizou's blog posts (where she consistently genders herself female), the various officially Furuya-endorsed artworks depicting her dysphoria in the Wayama Yama collab book, or the fact that in the gag anime even Zera gives her a gender-appropriate female bathing suit.
We know what a flamboyant gay boy character looks like in Litchi Hikari Club, because we've seen Jaibo. Raizou is nothing like him.
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