I think Anno's appeal is that she doesn't fit the mould of what we might expect her character to be.
She's not someone that you have to get past the defences to get to know the real Anno, slowly building up the relationship like it has been done so many times with other characters (both in TiTS and other games). You know the real Anno pretty much the instant you meet her; she's flirty, fun and friendly. There's no perma-angst-ridden backstory, sure she made a mistake but she forced herself to move on and she has.
I think the key moment in her story is when you have the shooting contest on Tarkus, if you beat her and just go for the Kind option of just having a nap and a cuddle, she's actually taken aback by that, expecting you to go for some fetish indulgence.
Despite all the fun, if you look closer at her, there's a deeper character. She feels second best to Syri, especially because Syri's an engineer on Mhen'ga working for peanuts but still does complicated equations in her head. If you take Anno to the Dorna Christmas party, she's taken aback that you choose to forgo tradition and ask her to dance.
In short, Anno is like so many interesting characters that have popped up on TiTS for the same reasons, you take the time to dig a little and you find something that clicks.
