Yegelos was the ancient figure in the legends of Contigo. Living in the world's ancient past, back when Contigo was part of a larger culture of Jupiter Adepts named the Anemos, Yegelos was something of a national hero to the Anemos. The legends remember him as a handsome man who was a natural warrior and loved to fight.
When the ancient wars of Alchemy were waged during this time period, the nation of Shaman from Hesperia attacked the nation of Anemos, who were led by the powerful mystic Hoabna, and the Anemos responded by sending Yegelos out to meet him in battle. The two figureheads waged many personal conflicts, one of which was in the location of modern-day Trial Road on Hesperia (and thus was the inspiration of Trial Road's creation), until some disastrous thing happened to make the fighting cease between the two nations. This event is stated to be connected to the event of putting out the beacons of the four Lighthouses and the subsequent sealing of Alchemy, and may have thus been also connected to the raising of the City of the Anemos to safety in the sky where it would become the Moon.
Following the end of the wars, Hoabna and Yegelos forged a great friendship, along with their respective nations, and to signify this they exchanged national treasures for mutual safekeeping. Yegelos entrusted to Hoabna a magic gemstone named the Hover Jade to Shaman, the Jade being an object laced with the Psynergy-based power of flight that the Anemos were famous for, while Hoabna entrusted a sacred staff named the Shaman's Rod to Contigo. They made a vow to each other: In the future, should a descendant of Yegelos return the rod to Shaman, the bearer will gain the Hover Jade.
It is often assumed that Ivan is a modern-day descendant of Yegelos. However, in The Lost Age, the individual that ends up fulfilling the vow between the two figures is Sheba, who is both physically similar to Ivan as a Jupiter Adept and appears very similar to Ivan in key aesthetic details, and the people of Shaman claim she must be a descendant of Contigo (and thus Yegelos) because she passes their tests. But just as Moapa is not certain Sheba should qualify because she's a girl, this indication that Sheba might be of Anemos heritage should be taken with a grain of salt. If Sheba is Anemos in heritage, it would have some implications for her own backstory.