I agree also how did she learn it.
Figarland genes
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My guess is that Bonney’s distorted future didn’t literally become Nika, she simply aged herself into a form where she has white hair, mimicking Nika.
As opposed to her doing something insane like getting rubber powers.
In OP, a lot of people seem to hit their prime around middle age. Yet Bonney’s main adult form is only in her 20s. Hence my guess is that Bonney’s strongest form is a middle aged form where she has white hair like Nika. As a result of being in her 40s/50s, this Bonney would probably be a lot stronger.