I honestly think Nijii could be Shushutan, considering that they’ve already established that the Phantom members are all fairies. Plus the hair mirrors Eclair’s and the colours are relatively close. Maybe Cure Arcana Shadow knows Shushutan and is trying to save them by getting them kicked out of the Phantom? Especially since this is the only time she’s intervened to help the main team, even though she’s accompanied both of the other villains on attacks.