This is Luteria (They/Them), my pretty Mephistopheles Tiefling Warlock! They are dating Karlach after a short relationship with Astarion.
Astarion and Luteria are still very dear friends as the breakup was agreed upon, despite the amount of crying Luteria did. (Luteria broke up with Astarion because they felt guilty about all the things they'd done together after Astarion told them about his trauma)
Originally from the lower city of Baldur's Gate at a very young age they were found to be greatly gifted in the magic arts. Initially set to become a cleric of Selune until one night their families village got attacked by rogue group of Necromancy Cultists.
Luteria then made a pact with their current patron which granted them powers to save the village and drive the cultists out. Much to their parents dismay, Luteria then set off across Faerun to become an adventurer/mercenary before they then got abducted by the mindflayers which then goes into the events of the game itself!
They still very much have love for Astarion and have spoken to Karlach about it in great detail (Karlach wouldn't let the topic go because she could tell it was bothering Luteria). Karlach has stated on multiple occasions that if/when Astarion is ready/comfortable, he has Karlach's expressed (and rather enthusiastic) permission to ask Luteria to be his partner again <3