Regarding Maddie Nolen
We are introduced to Maddie Nolen after Vi turns down Caitlyn's invite to join the enforcers against Jinx. We see her getting heavy with the Booze alongside Loris, another character in Cait's fire squad. Maddie's story of Caitlyn's fight to get Vi in sparks interest in Vi, who eventually does join the fire squad with her hex gauntlets ("overdesigned bitch mittens").
We then see Maddie again, getting closer to Caitlyn after CaitVi's last breakup after the ambush on Jinx goes wrong due to Isha. It's clear she's gotten closer with Caitlyn, serving as a confidant and a friend. This is shown further by Ambessa's comment about seeing her around more often.
Of course, we find out that Maddie is a Noxian, loyal to Ambessa, who ends up shooting Caitlyn (and dying because of it). The fandom is joyous at this, stating that they never liked Maddie. I understand why, since CaitVi is ride or die! However, even though she stood in the way of Cait and Vi's relationship, I didn't really hate her. I thought her accent was adorable ("Junior officer Nolen! ...Maddie."), and I think it was interesting for plot reasons.
I'm unsure if this is a widespread opinion, but I've seen some people say that the "twist villain" that Maddie turned out to be was... unnecessary. While I think they ought to have done it better, I think it could have been a really great way to show just how far Ambessa was willing to go to take of Piltover. She had a young Noxian officer join the enforcers undercover and court Caitlyn. Ambessa knew that Caitlyn was in an emotionally fragile place, and she took the opportunity. Whether or not Ambessa knew that Caitlyn was queer or not, she had clearly instructed Maddie to get close. Whether it was Maddie or Ambessa's choice to go for the pussy is unknown. However, due to the line, "I did appreciate your warmth," I have reason to believe that Maddie chose to interact sexually and romantically with Caitlyn (after all, Caitlyn has a devastating face card and an even more devastating body card). Furthermore, this is proven when Ambessa tells Vi that she would've been a valuable asset if she wasn't a distraction to Caitlyn, showing that she'd rather have Caitlyn being romantically involved with someone who answers to Ambessa.
So, what was Maddie's purpose? Did you know they almost scrapped her character and recast due to COVID-19?
I don't believe it was just a "cheap" way to add another block between Caitlyn and Vi or a cheap way to get rid of Maddie after having been romantically involved with Caitlyn so that CaitVi could triumph.
Instead, I believe it shows just how far the Noxians were willing to go. She was willing to get close, even sexually/romantically involved, with Piltover's commander in order to keep close tabs on her, likely by Ambessa's order. If Arcane's writers had been given more screen time, they likely could have fleshed out this relationship. Viewers dislike it because it seems random and sudden. However, many people have experienced the feelings of needing a "rebound". Maddie provided what Caitlyn had lost in Vi, so it's natural that in her grief and emotional instability that Maddie was able to manipulate her so much.
Although, it's possible "undercover Noxian" had been Arcane's plan with Maddie Nolen all along, it's not very clear due to the rushed writing. It's very possible that Maddie suggesting Caitlyn rule as her own person rather than under Ambessa was either a hint that they actually hadn't intended to make Maddie a Noxian or subtext that Maddie was undercover and was trying to test Caitlyn. Both options are very possible.
There's something really fucking funny to me about how Maddie took off her mask dramatically to reveal who she is, as if we didn't immediately know from that fuckass orange bob.
Lastly, "I did appreciate your warmth" before pulling the trigger is quite badass, all things considered. Imagine being told you were a good fuck before getting your brains blown out. The part that really gets me on Maddie's character is her choice to aim at Caitlyn's neck rather than her head. Both would kill Caitlyn surely, but aiming at the neck was a choice that was purely Maddie's. Aiming for the neck is the recipe for a slow, agonizing death. Caitlyn would have been paralyzed and bleeding out, slowly dying as her heart and brain fail without each other. She made the conscious choice to inflict this death on Caitlyn rather than just giving her a quick and painless death. All I can say is that this point for Caitlyn is absolutely devastating as she sat there defeated, her allies captured or dead around her, and betrayed by someone she thought she could trust.
In conclusion, Maddie's character was a little rushed. The idea was there, but the execution (no pun intended) was lackluster. It felt almost like an afterthought. If they had given more time to flesh out her character and give us a reason to root for her (other than "you should like her because she's screwing Caitlyn!"), the betrayal would have hit harder and been more exciting. But alas, it felt more like the Zootopia ending where we find out that the lamb was the villain all along.