Lup

"I believe that, one of these times, we’re gonna get it right. And we’re gonna find a way to defeat The Hunger and... save everybody inside of it. I have to believe that, to keep doing what we do. Because I have to believe [choking up] that I’m gonna get...those fifteen dollars back from Greg fucking Grimmaldis!"

Lup Tacco is the twin sister of Taako and the creator of the Phoenix Fire Gauntlet and the Umbra Staff.

Early Life
Lup and her brother, Taako, were passed around among family members, never sticking with any for long, after the age of 12. After years of moving around, 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.

As the storm starts to consume their world, the crew catches sight of The Hunger. They decide to evade and not return until after the danger has passed. When they touch back down, they realize they're not in the same plane they left.

Cycle One
After the Red Robes land onto The Animal Kingdom, they see The Light of Creation crash-land somewhere on it. Being good at languages, the twins spend their first year talking to the animals and learning history in The Animal Kingdom. The first animals they are able to speak to is a family of mongooses. The animal language is only learned by Taako, Lup, and Barry Bluejeans. After Merle and Davenport find its location, the crew goes with Magnus to try to gain access to The Light of Creation. After causing a scene, Magnus and the rest of the crew enter the animal's High Court.

After a year of negotiating and gaining their trust, the Red Robes almost manage to get The Light of Creation from the High Court. Unfortunately, The Hunger attacks The Animal Kingdom before they can retrieve it, and they are forced to make a hasty retreat, with Taako and Lup being the first to flee. While Taako bolted without heeding the status of the others, Lup took the time to take potshots at The Hunger. As they leave, they watch The Hunger become stronger as it absorbs The Light of Creation.

Cycle Eighty-Two
Lup and Barry decide to become liches so they can become strong enough to defeat The Hunger.

Cycle Ninety-Two
Lup and the other Red Robes study artificing at the Hanging Arcaneum. She creates the Umbra Staff and devises the design for the Phoenix Fire Gauntlet.

Cycle Ninety-Nine
Lup creates the Phoenix Fire Gauntlet and puts it out into the world. Over the following 20 months, it is used numerous times in the Relic War, turning entire towns into circles of black glass. The devastation wrought by her creation weighs heavily on Lup and one day she simply disappears. She leaves behind a note saying only "Back soon" but even as time passes and the others search for her, she is nowhere to be found.

What the others don't know is that, in an attempt to hide away the Gauntlet in Wave Echo Cave's vault, she was killed by Cyrus Rockseeker, a member of the Rockseeker clan. He stabbed her with a Silverpoint-laced knife in order to take the Gauntlet for himself. Before succumbing to her wounds, she would manage to lock Cyrus away in the vault, trapping him inside. She was not able to escape, however, as her lich form was sucked into and trapped in the Umbra Staff.

Occurrences 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
When Taako equips the Band of Projected Thought, he hears a woman's voice that makes his heart swell, he knows this voice belongs to someone special to him but he cannot remember who. The voice tells him "Trust Barry. Love Barry. Taako, it’s - it’s me, it’s [static] it’s me, trust Barry."

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, and transitioned when she was fairly young.
 * 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.
 * Lup has been nicknamed "The Vore Twin" by parts of the fandom, referring to her umbrella consuming wizards.
 * Lup is in a romantic relationship with Barry Bluejeans.