
Based on my limited data, in animu the only purpose of a little sister-type character is to provide incestuous overtones.
“Sempai’s little sister” is the next rival character. The original idea was that she would be the cute and innocent type, but the notion sort of changed. A summary of the discussion was: “a lewd/jailbaity version might make you feel a little less bad about offing her”
All designs subject to change, etc.
One of the “rival” characters. She’s the “generically sporty” girl, as we on the team have called her. She probably won’t wear her jacket like a cape in later versions. Maybe she’ll wear it as a scarf, or around her waist. Since she’s more athletic/strong than the player character it’ll probably be more difficult to deal with her, at least in a physical/combat way.
As always, no design as of yet is finalized.

Dunno if the scarf will stay, but if it does, in a game like this, a scarf could theoretically be used against you.
First up is the newspaper club girl. She’s the “infowoman” character that you get data on your rivals from. I think Yanderedev mentioned how she’s a sidekick of sorts, so maybe she’ll be able to help you out somehow during gameplay.
Anyway, none of the designs are final, so if you have problems with any of them, you don’t have to lose sleep over it.

I think the three hairstyles in the middle were based off of edits people posted on /v/, but I don’t remember which.
For posterity’s sake, here are some of the hairstyles that were proposed for the playable character that I posted on /v/ back whenever that was. This was before we had a 3d modeler on the team, and we were assuming that we wouldn’t have the resources to build models for character customization.
As of now, we do in fact have a 3d guy on the team, so as far as I know, there may be some degree of customization for the player character.
None of the designs you see, or will see are final. In fact they probably won’t be final until you actually see them in the finished game.