can I ask why you want Ada to get redemption arc? because of the way she's been written, I feel like her appeal dies if all of a sudden she's Leon's secret partner who uses her previous merc knowledge to help out the good guys and is most likely sanitized by proximity of Leon's inner circle
To be honest, I think Ada already has a redemption arc going on. I also don't think it flattens her character in any way; it actually adds to it.
Ada has been shown to have strong moral impulses since RE2. I also don't like the notion that these impulses are due to Leon's influence alone. Obviously Ada does care for him, but her protection of Leon is only one example.
When Ada is first introduced in RE2, she's honestly a pretty standard mercenary character.
She shows up in a trench coat and sunglasses. She has all the quintessential traits of that femme fatale spy who is always up to something shady. This alone is not a particularly interesting or deep character. We all know this trope. However, that isn't all that Ada is.
Ada did not have to save Leon here. She could have left him to die and chased after Annette instead. But she didn't.
Then here, we have a more obvious example. You can see the conflict on her face. This is where we start to see that there is something other than a paycheck motivating Ada. Despite her attempts to pass herself off as a cold, apathetic mercenary, she cannot bring herself to harm Leon. Even when he is the only obstacle to her completing the mission.
I know it's easy to look at this and jump to the conclusion that these decisions are because of Leon's pure moral influence on her... but I don't think that's it at all. This is Ada's own moral compass guiding her. She cannot bring herself to hurt someone who she knows to be good and kind, who risked everything on the chance he could save lives.
And while obviously her actions at the end of RE2 show she had nefarious motives all along... this is actually the last time we really see her actively assisting a villain.
Look at how she deals with Wesker at the end of RE4 (Separate Ways). This is another moral impulse. Ada has seen what widespread bioterrorism does by now. She knows what she has is powerful, and contradicting her opening monologue... she actually does ask who or why this time. She wants to know what Wesker's plans are, and she betrays him immediately after he tells her he's planning a mass genocide. Again, I'm not going to simplify this to Leon's good and pure influence over her evil mind or something. No; this is all Ada. Leon was already safe. In fact, he probably left that island thinking she betrayed him again. Ada did this herself.
Now consider RE6, her latest canon appearance.
Ada is not in an antagonist role at all here. In fact, her whole story here revolves around working to take down Derek Simmons, a former employer who had a creepy obsession with her. And it is incredibly gross, creepy, and sexually charged... just listen to Simmons' dialogue during the boss fight. This is significant because it's showing Ada facing the demons of her past. She is facing the same enemy that the heroes face in this game with her own independent motive.
And to sum up my point, I think everything I just said gives Ada's character much more depth and intrigue than her simply being a spy who never changes, only moves on to the next bad guy to work for. Ada is a lot more than that. We can already see her being "redeemed" in a sense. We see her display moral impulses. We see her growing disdain for former employers, including Wesker and Simmons. Ada is already on that path of redemption.
While I do ship Ada and Leon, I don't support the notion that Leon is responsible for all these changes in Ada. He definitely is important in her life; Ada seems to admire Leon's compassionate heart very much and I do think it's what she fell in love with about him. Leon is a factor in Ada's story, but not the only one. Ada has her own complex journey going on here... and I like the idea of that journey proceeding to the conclusion I see it pointing in. I like the idea of Leon and Ada's stories converging to a point where they are both able to put the professions that caused them so much trauma behind them.