In my opinion str should be for every character that's it's no the MC, because sometime it's hard do tell if a character is NPC or SC.
After a little bit of research ...
To determine the main character, we need to understand
the subject of the story, as well as from
who's POV (Point Of View) when the story progressing. Because almost all VN games in the making, writers can switch the subject at any times. But if the creator didn't create major changes, then it will be easier to determine lead character, supporting character and minor character. From there, you can accurately tag the game. I have to correcting about Support Character. They are not the same as Lead Character. Support character still consider NPC in games.
In
Dead Moon Survival : the subject is the family survival. The boy, sister & mother have their own POV. Despite only the boy & the mother who have decision to choose what course of the game, it's irrelevant as the story telling focus heavily on the family. They all main characters. So if the mother fuck the sister, it become [true
M/D],
not [str true
M/D].
In
Dream of Desire : The subject is the Son discover magic. Despite the mother, aunt, sister have their own POV when dreaming, it's only to support the narration. Only the boy are lead character, the other are supporting. If the mother fuck the sister or aunt, it become [str true
M/D &/or
S/S].
In
Dating my Daughter : The subject is Daughter & Father reunion. Even though most of the time you play the father POV, the daughter POV also actively driven the plot. Both are main characters. The rest are supporting and minor characters. If the daughter fuck her mother, who's not lead character, then it's [true
M/D], not [str true
M/D].
In
Where the Heart Is : The subject is the Guy reconnect with close family's friend. Hardly any POV from the other characters, all the female are supporting and few minor characters. If the mother or any sister fucking each other, it should be [str true
M/D &/or
S/S]
Problem is, to determine a non lead character, but related to MC. They are not stranger as it's literally meant. But for tagging, they should be considered [str true], not [true].