Penelope

Penelope is a bionic adventurer who lives in the far future with her magical cat companion. She was first seen at the end of Volume 6, retrieving Carl the Gem and Gemma the Gemstone from Finn and his friends' dungeon. Later, at the end of Volume 7, where her name is revealed, she entered the ruins of Princess Bubblegum's library hoping to find something that would help her fight the Awful Offal Abominations plaguing the land, although all she found was a comic book released by Marceline.

In Volume 11, it's revealed that she is a female, future version of Finn.

In Volume 13, Penelope is transported to the moon and obtains the Moon Sword, and because of her bionic body she can use the sword without stopping when she feels pain. She returns to a greener future where she is on a quest to find Bubblegum. It's also implied that her tampering with space-time may have freed Gata, though it is unknown if her mother's powers would have failed even if Penelope hadn't altered reality.