Mentor is the leader of the Titanian colony. His back-story was based on Greek mythology. Some years later, he was retconned to be a member of the Eternals, a race in the Marvel Universe created by Jack Kirby, separately based on Greek mythology.
A'lars and Zuras, both adults at the time of Chronos' death, held a general assembly to determine who would succeed him. The assembly chose Zuras, so A'lars chose to leave the colony to avoid the sort of fraternal rivalry that led to the Chronos and his brother Uranos to the first civil war. He settled on Titan, taking over as leader of the underground colony that his uncle Uranos had built centuries earlier.
Upon arriving on Titan, A'lars discovered the Uranian Eternal Sui-San, the sole survivor of the civil war that destroyed the Eternals of Titan previously. Sui-San bore two children for Mentor, Thanos and Eros (later known as Starfox). Sui-San died in Thanos' first major attack against Titan.
Mentor implored his father Kronos to aid Titan against Thanos, causing Kronos to create Drax the Destroyer.
Mentor later met Rick Jones and Captain Mar-Vell. With his son Eros, he informed Jones and Mar-Vell of Thanos's degradation of Titan. Mentor enlisted Mar-Vell's aid against Thanos. Mentor was imprisoned by Thanos, but freed by Captain Marvel. Mentor joined in the battle against Thanos. Later, Mentor joined in the attempt to cure Captain Mar-Vell of cancer, and attended Mar-Vell's last hours.
Mentor later informed the Silver Surfer of Thanos's history.
Mentor has knowledge of science and technology far in advance of present-day Earth. As ruler of Titan, Mentor has access to and full use of a wide range of advanced technology located on that artificial moon.