Everything already mentioned here. She's one of the few characters in the series with any kind of swag. Yeah, she's a generic femme fatale character. But its better than most of the other females. Most other RE females fall into generic cute white girl.
And add Ada's theme from RE2. That thing is awesome. --- "No man should fight any war but his own."
I've never liked Ada. In 2 she was just some spy that nearly got the others killed and worked for The Organization which is arguably as bad if not worse than Umbrella in ways. Plus she could arguably be seen as the first of the whole "super secret agent" crap that the series turned into. --- Before you die you see the Tails Doll---Backdrop Observer of the Metroid: Other M board Apparently Edward: Wise Old Sage of Korodai