Lup

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

The Stolen Century
Lup joined the Institute of Planar Research with her twin brother, Taako, after reaching adulthood, presumably after the age of one hundred to become part of her world's first group of explorers. They acted as both chefs and arcanists during the expedition, being sources of magical information.

Occurences Afterward
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 "Chaalupa". It was later confirmed in the second The "The Adventure Zone" Zone that, while this was the original joke, Griffin feared it would be seen as insensitive, since fans often portrayed Taako as Latinx. It is now canonically not short for anything.
 * Lup is a trans woman.
 * Greg Grimaldous owes Lup $15, which she more than likely will not receive seeing as how her home dimension, which is the dimension in which Greg Grimaldous lived, was destroyed and eaten by the Hunger.