Who is Xanatos and where does he come from?
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Xanatos officially appears in the Jedi Apprentice series by Jude Watson, published at Scholastic. He physically has his entrance in JA #2, The Dark Rival. This also gives you most of the information on him. In the following books, he's merely mentioned. He reappears in JA #7 and apparently will have his great entrance in JA #8 which is due to appear in June.

What does he look like?
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In JA #2 (p.21) he is described: "A young man stood in the doorway. His shiny black cape was lined in a blue so deep it was almost black as well. A broken circle scar marked his cheek. [...] Xanatos strode forward, handsome and commanding. His black hair flowed to his shoulders, and his dark blue eyes matched the lining of his cape."

Xanatos is also on the cover of JA #2. He is in fact quite handsome, almost pretty. The scar is on his right cheek.

He still has his lightsaber which is red.

How old is he?
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There are no specific details in the books, but we assume that Xanatos is about 10 to 15 years Obi-Wan's senior since he turned at the brink of becoming a Jedi knight which places him around the age of 20 then. The novels give the impression that only a few years have past between Xanatos' turning and Qui-Gon's choosing Obi-Wan as his Padawan learner. Thus it can be assumed that Xanatos is in his mid to late twenties at the time of JA.

Why did he fall to the Dark Side? ¹
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Greed for power, wealth, material things. Also, there is a hint that he resented Qui-Gon's coldness toward him. In JA #2, he meets Obi-Wan and commiserates about the fact that Qui-Gon never opened up or trusted him. This deepens Obi-Wan's frustration with Qui-Gon refusing to take him as Padawan at the time.

Xanatos fell at the cusp of his becoming a Knight, when he and Qui-Gon undertook a mission to Telos, Xanatos' home planet, where Xanatos joined with his father, the power-mad governor Crion, to raise an army and attack the neighboring world. Qui-Gon stopped him and killed his father, severing the ring from Crion's hand.

Xanatos picked this ring up and seared his face with it, thus creating the distinctive and emblematic broken circle scar on his right cheek. He hated Qui-Gon and the Jedi and blamed them for his father's death. He then took up the reigns of Telos and has pursued mercenary ends ever since.

What kind of personality/character does he have? ¹
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He's quite haughty and polite at the same time, a somewhat overbearing mid-level management sort of person. However, he has deep resentments toward the Jedi and Qui-Gon in particular. He concocts the mission to the mining planet Bandomeer, a complex affair, for the sole purpose of destroying Qui-Gon with as much pain and agony as his father must have felt at his death by Qui-Gon's hand.

Also, Xanatos is quite clever with gadgets and technology. He has designed an electromagnetism triggered wall, which is opaque until a modicum of Force is used. Then it turns transparent. He uses it to hide his private office on Bandomeer. He is fond of saying "I always have a back door", an escape or way out of everything he plans.

What exactly does he do in JA?
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When Qui-Gon and Xanatos meet again, on the planet of Bandomeer, Xanatos is the envoy of a cartel-like mining company called Offworld Corporation. He acts as if nothing had ever happened, is amiable and dashing. But he soon shows his true face. In fact he is the owner of OC and even more ruthless than he ever was before, willing to kill everybody, even blow up the whole planet, just to increase his wealth and power and to take revenge upon Qui-Gon. He lets Obi-Wan being kidnapped and brought to a slave ship. When Obi-Wan finally manages to escape, Xanatos maneuvers him and Qui-Gon into a mining tunnel, planning to have them killed in the explosion of the bombs he has set all over the planet. He is able to make a getaway before the Jedi can make him pay for his crime.

Then Obi-Wan and Qui-Gon are sent to the planet of Gala to make sure that democratic elections take place. Whereas they don't encounter Xanatos in person, he still has his share of the problems they face on this mission. On a planet gone mad where several parties fight for winning the election, Offworld Corporation is in league with Deca Brun, one of the candidates, bribing him by sponsoring his campaign while planning to exploit the planet's natural resources in future mining.

Xanatos has his next entrance in JA #7, but I didn't read that one yet, so please bear with me! ^_^