Are you sure there was a character named Ripley? I just went and looked at a listing of characters https://james-camerons-avatar.fandom.com/wiki/Category:Chara... and they didn't have Ripley, but they did have Dancer #3, so I figured that's a pretty authoritative list.
Maybe you mean Grace Augustine, Sigourney Weaver's character?
on edit: I mean it's a pretty hilarious confirmation of my point if so.
I actually liked the movie, it's just that the characters aren't often addressed by names, so it's hard to remember them. In Aliens, for example, people always refer to each other by names (Vasques, Hudson, Bishop, etc), but in Avatar they don't.
(And i can probably name actors who played most roles, including Giovanni Ribisi)