Lup

Lup is the mysterious twin sister of Taako and the creator of the Phoenix Fire Gauntlet.

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.

It is also insinuated that there was a relationship between Lup and Barry Bluejeans.

Trivia

 * Popular fan theory is that "Lup" is short for "Chalupa". It was later confirmed in the second The "The Adventure Zone" Zone that, while this was the original inspiration for the name, it is canonically not short for anything.
 * Lup is a trans woman.