-Chan is very diminutive and might not be used depending on the female character in question. I've heard female characters referred to as -kun, albeit more rarely, but yes. -San might be a better suffix to use for a female. -Kun is considered more masculine.
Along with being diminutive, it's also rather gendered in usage -- a man or boy is generally not going to call another male -chan unless it's a very small child, but girls and women who are very close friends will call each other -chan even into adulthood. So for Red's dojo setting, there could be female students addressing each other with a mix of -san and -chan, depending on whether the characters are old friends who've known each other before taking the classes, or new acquiantances who just met at the dojo. -kun can be used for a girl who's a bit tomboyish -- I've also read reports that you're starting to see some teachers and supervisors using -kun for all older female students or female employees, trying to get away from the diminutive/gendered implications of -chan, but apparently that's not yet the norm.
(Also in a close family situation, -chan/-kun may also get used as a sign of endearment and intimacy for people well past the age where it would seem appropriate. Parents may continue to refer to their children as -chan/-kun after they've grown up, and grandmothers and aunts may be affectionately called obaa-chan/oba-chan instead of the more formal -san.)
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(Also in a close family situation, -chan/-kun may also get used as a sign of endearment and intimacy for people well past the age where it would seem appropriate. Parents may continue to refer to their children as -chan/-kun after they've grown up, and grandmothers and aunts may be affectionately called obaa-chan/oba-chan instead of the more formal -san.)
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