She constantly has resting bitch face, isn't as well endowed as the other characters and in general just has nothing that really stands out about her looks (Nothing attractive, the face does stand out).
We already have the bitchy girl, it's vicky and maybe a bit Alyvia. But that's also it, her character doesn't stand out either. Alyvia has her romantic side, Vicky has the classic tsundere juxtaposition of her aggressive side normally and the shy, embarrassed side when she see's glimpses of mc's younger self in mc. And Mya has that playfull, flirty personality that is endearing.
Melana is just vapid. She just wants mc because he's the current talk of the school among the girls. And she is annoying, but not in a bratty way that would be fun to tame. Vicky will be fun to tame, Alyvia will be fun to conquer, Mya will be fun to dominate. Melana has no real appeal in that way.
If you disagree then great. Good for you. But it's not gonna happen, the dev introduced the whole leo and melana romance so that he has an excuse to have leo be absent. He already has so many storylines and LI's to juggle, he's not gonna introduce a whole other storyline just to add another LI and put Leo into the forefront of the plot after he went through the trouble to have less of leo, not more.
We already know what the last secondary LI is gonna look like and it's gonna be a milf. You can find her in some pictures already. I think she was in the last christmas screenshot baibai posted.