[Tenzen12]

She came from nowhere alright, but hardly like "just another heroine". And yes maybe girls that are in love with MC as default usually doesn't win, but she is winning pretty decisively right now already which is unusual in first place. If it doesn't follow usual formula now, why you think it will later?

And as you said Chihiro did happen appear in chapter three, but let's be honest it would change nothing if it was fourth one instead as there were zero plans for her at that time anyway. Her winning is unrelated to chapter count. Same for Chisame. You are speaking about what happens usually but choosing side characters over main heroines is much more unusual. And of course we have there adaptations of LN, like Kampfer where winning girl doesn't appear until ...like episode five? Should I present more examples from LN territory?

In anime related medias and story telling in general there there are no absolute rules. That's why you have to inspect every work by it's own merit, because there will always one or two ready throw curve ball at you . If you don't know that much, it means you didn't read/watch enough stuff yet.