I have always considered Ada the female Wesker, her voice is cool and matches her very well, and she is sort of a villian, but not really, they are very similar those two, which may be why they just don't mix, as opposites attract.
Sally Cahill voiced Ada in both RE2 & RE4 and did such an amazing job, with the character's development also just as great.
I always liked her in RE2, she played a character that was playing a civillian, and the way the plot all went down was perfect. This is one reason that I dislike her so far in DC, as she is wearing her RE4 Mercs outfit, which isn't what she looked like in RE2. The dress is it, very late 80's early 90's type of style looked great, but in RE4 it wasn't meant to be an exact remake, it was intended to redefine her original appearance for the sake of an alternate costume for the Mercenaries minigame.
So her original slippers and sweats(or whatever they are) were replaced by high heels & pannyhose, which that alone is stupid on Capcom's part, but what really takes the stupidity to the peak is how she still has her gloves and gun holsters on, I mean seriously, why not just burn "SPY" into her forhead....
In UC her voice actress was different, and as you could imagine didn't sound so great, as well as the same issues with her outfit, it is still the RE4 Mercs one, with some tears in it.
Also you may have noticed that her hair went from dark brown in RE2 to jet black in RE4, no big deal, but when we are seeing her hair as jet black in the RE2 stuff of UC & DC, that is when it gets annoying, damn Capcom is so incredibly lazy......
But anyway, let me conclude by saying that she was, and hopefully still will be a great character, I give her a 9.5/10.