Alison has a literal crush on Ian, its clearly with almost every interaction with her.
She openly gets annoyed if you ignore her for other girls of the group. If she's with Jeremy, she implies that she does not find Jeremy interesting and only hangs out with him for the sex (and implies that she would not be with Jeremy is Ian had been more attentive towards her). She is clearly annoyed how everyone seems to associate Ian with Lena. She is also open to change for Ian, you can already see that she changes her fashion sense.
Technically, Ian and Lena should be the least involved couple. The dev wants to make Lena's scenes hot and diverse, but at the same time also wants Lena to be realistic and grounded (which doesnt properly work imo).
TL;DR Alison is the true LI of Ian, and maker her thiccer puts her at a tier similar or above to Lena.