Altos was a young dark haired man sent by Arbitan to find the keys to the Conscience of Marinus. Instead his friend Eprin and he were enslaved in Morphoton thanks to power of the Mesmeron.
As a Morpho's slave he welcomed Barbara in the city, offering to grant her every wish. He did the same with Susan and the First Doctor, who respectively asked for a fancy dress and a complete visit to the laboratories. He assented to both their requests. Ian did not trust him because his eyes never blinked during the conversation. When Barbara proved herself resistant to the power of the Mesmeron, the Morphos ordered Altos to kill her. He tried to strangle her but Sabetha hit him over the head with a stool. He collapsed.
After being freed, Altos accompanied the Doctor and his companions in their search for the keys in the city of Millennius, showing the Doctor how to adjust his travel dial. He encountered Vasor, who tied him up in the forest in the company of the wolves and abandoned Sabetha and Susan in a cave. He was freed by Ian and helped him save Barbara from Vasor, who was trying to assault her, and rescue Sabetha and Susan in the cave protected by the Ice Soldiers.
Sabetha and he were romantically involved, which was exploited by the Voord Yartek, who forced him to hand over one of the Conscience's keys. (TV: The Keys of Marinus)