Flare, you definitely won't win over all of them :
- Setsuna : She will never join her side. Keyaru never brainwash or use drug on her. Light gaslighting at best, she follow him on her own free will.
- Kureha : 50/50 for me. He started with her with drugs, lies and manipulation, so if made her learn all of that, she had a chance to defect, but she also saw that the kingdom she sworn and fought to protect was worst, and the Keyaru did change it for the better. She also from real close, Keyaru heroic deeds.
- Ellen/Norn : As Ellen, no chance, even with learning the truth, she can conclude they were happier brainwashed. As Norn, she would have beef with Keyaru, but also feel gratitude since when brainwashed she could finally be happy with her big sister.
- Demon lord Eve : he indeed manipulated her and gaslighted her at the start, but he also always stood by her side even in the toughest moments. Little chance she leaves his side. At best she leaves, she will never join Flare side instead.
- Guren : Even as young and Naive as she is, she knows her dad is not a good person, know how scary he can be, and knows Flare's side won't be the winning one. No chance she joins Flare.
So at best Flare, Norn and Kureha against everyone else minus Eve, and at worst, just Flare against everyone else.
Yep, Keyaru won't lose that one, you would have better chance decapitate him in his sleep after the usual night snusnu.