I was thinking about this today, and I think you are onto something. Every other LI has a major characteristic, whereas Annie's is absent or at least a little lacklustre.
Alex comes from money and has her issues with her family (also tattoos), Nova is the ginger hacker with Dione, Luna is Mexican with a nice family and special seer powers, Penny is the blonde bombshell who loves to tease and is Instagram famous, Dalia is the gym rat with a nice ass, Nancy is the MILF, and Annie is...
Cute? Well, they are cute in some way. Innocent? So are Luna, Nova, and Dalia. A childhood friend? So are Dalia and Penny. She doesn't really have a powerful, unique X-Factor. It doesn't have a be a negative thing like addiction, but something that gives her a little USP that doesn't fill the same role as the others.
Don't get me wrong, I love Annie and wouldn't change her, but I can see why she might be a little unexciting to others.