Guys, people on other forums are saying this was a scrapped character. Named Cyan. She was gonna be introduced in FireRed &LeafGreen as Leaf's (Blue's) Rival if you picked her.

In the pokespe (Pokemon Adventures/Special) manga she was Green's (male) childhood friend. Which explains her role as a rival I guess.

Green was gonna be just Red's Rival, & Cyan was gonna just be Leaf's Rival, basically.

Also in the pokespe manga, there was gonna be her own chapter, and she was gonna have a sub-plot of her crush on her best friend, aka, Green.

She is mostly seen with her main Pokemon, Raichu.

In HGSS, if you picked the girl character (Lyra) Instead of having Green as the last Kanto badge, you'd get Cyan with a slightly different gym look.

In her game counterpart, She was gonna have a serious personality, kinda like Red's personality in the games, he silent. Though she wouldn't be silent, I guess she'd be reserved, but competitive.

Also in the games, her text was sometimes gonna mention, Green.

Sadly, she was scrapped. The Pokemon staff kept thinking of introducing her back, but they never did.