The Xliactian Historian is an alien who discovered the location of the Gargantis Array and is the only one who knows the jump codes to get there. She was first mentioned by Vortian Prisoner Number 777 to Zim in Issue 1, later appearing in the flesh in Issue 2.
She is currently in hiding, working as a gift shop owner at a store called the "Shmoop Shmop" on the Universe's Biggest Ball of Shmoop. She swore to keep the location of the Gargantis Array a secret in order to protect it from people who would use it for evil.
However, it's actually fairly easy to convince her to reveal the jump codes just by purchasing lots of souvenirs from her gift shop, as both Zim and Dib find out.