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There isn't any proof that Fionna has a last name yet. Judging the fact that she is the gender-swapped version of Finn, she might inherit farm-world Finn's last name, which is Mertens. However, both Finn and Fionna are considered Finn/Fionna the Human, so there title could probably also serve as a type of last name maybe.

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11y ago

What else can I help you with?