Xanatos is a human from Telos IV who trained as Qui-Gon Jinn's padawan and fell to the dark side, introduced in Jude Watson's Jedi Apprentice: The Dark Rival (Scholastic, 1999), the second book in the series.
He is the load-bearing piece of backstory the whole Legends Jedi Apprentice line was built to supply. The Phantom Menace gives no reason why a Jedi as capable as Qui-Gon is reluctant to take an apprentice; Watson's answer is a previous student who was brilliant, who was the son of a corrupt planetary governor, and who chose his father's power over the Order when the two were put in front of him.
What follows across the series is not a duel but a stalking. Xanatos runs a mining corporation, buys influence, recruits Bruck Chun inside the Temple itself, and keeps arranging situations in which Qui-Gon has to choose between the mission and the boy. He is a villain built for serialised children's fiction and he works on exactly those terms.
Current canon has no Xanatos. The gap he was invented to fill has since been filled differently — Dooku, in Dooku: Jedi Lost, now carries much of the same weight as the master Qui-Gon could not save. Both answers are good; only one is binding.
