Maybe it's usagis long lost sister lol

This is a very good question because studio was released before club which means Usalina wouldn't be introduced yet but neither would Mily.

Edit: meant Usalina, not Usagi

Usagi was introduced in Anime Gacha so she has been here since the beginning

She has?

I haven't noticed.

Oh, I meant Usalina. Misstyped.

Usalina's character design doesn't match the character but Milys character design does very closely. Could be Mily was intended to be introduced before studio but Lucas forgot or rushed or something.

That would make sense. Though I can't find any conformation. I also checked the artist page but found nothing.

Who even was the artist?

Alonair

It might be an early/alternative appearance or design of another character.