Lup

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

Early History
Lup and her brother, Taako, were passed around between family members, never sticking with any for long, after the age of 12. After years of this, they learned to only trust each other.

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.

Before leaving their home world, the crew goes to a biker bar to party and fight. Taako and Lup spend their time hustling people at pool. They take the losers' magical shoes since cash is worthless to them at this point.

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, fans often portray Taako (and by extension, his twin sister) as Latinx, and Griffin thought that this name might be misconstrued as racially insensitive. It is now canonically not short for anything.
 * Lup is a trans woman.
 * Greg Grimmaldis owes Lup $15, which she more than likely will not receive seeing as how her home dimension, which is the dimension in which Greg Grimmaldis lived, was destroyed and eaten by the Hunger.