Lup

Lup is a mysterious character who has been mentioned a few times.

Occurrences (contains spoilers)
The Red Robe mentioned her by name in the final episode of The Eleventh Hour ("Lup, they don't trust me. I can't do it anymore, Lup, I'm sorry."), and yelled "You found her!?" in episode 7 of the Crystal Kingdom after he saw the Umbra Staff. From the Red-Robe's reaction and from the time the Umbra Staff burned "LUP" into the wall in Lunar Interlude III, it's implied that it belonged to Lup.

Lunar Interlude V revealed that Lup was the creator of the Phoenix Fire Gauntlet, and the Red Robe has been searching for her for at least a decade. Her connection to the Red Robe, her connection to the Umbra Staff (first found in the grasp of a red-robed skeleton), and her role in creating a Grand Relic all insinuate she is a Red Robe, too. Part two revealed that she is Taako's sister.