Dr. Spector (full name Johann Schleimann Spector) is a friend of the Avatar on Earth, and as his title suggests, an academic. He also is an archaeologist and adventurer.
Spector was thrown to Eodon some time before the Avatar, with his assistant Fritz. He went insane after an encounter in the ancient city of Kotl, starting to have world-conquering plans, which included putting the usurper Huitlapacti on the throne of the Nahuatla Tribe by granting him invulnerability.
In Savage Empire, Dr. Spector was at first going by his Nahuatla-equivalent name, "Zipactriotl" (pronounced Zee-pack-tree-OT-tl). The Avatar had to restore Spector's mind by shutting down the control panel in Kotl, before he would help on the quest (prior to this, the insane Doctor was actually a threat). After the Myrmidex threat was ended, he returned to Earth along with the Avatar.
Spector also stood at the Avatar's side in Martian Dreams, after they both received a mysterious package that showed a photo of both of them in the past. He was very helpful in the quest on Mars 100 years in the past, being the brains of the party. He returned to the present with the Avatar once their mission was completed.