Lup

Lup is the mysterious sister of Taako and the creator of the Phoenix Fire Gauntlet. Popular fan theory is that "Lup" is short for "Chalupa" but this has not been confirmed.

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 Part One 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.