You might consider Eidolon a sort of loose mixture of the character Pariah from DC's Crisis on Infinite Earths and Belgarath the Sorceror. As the one responsible for Hearthstone's "original sin" and fall (we'll tell all about the details in a future project), Eidolon has lived for over forty thousand of Hearthstone's years, and has a unique place in its history as a prophet, priest, and stemmer of the tides of iniquity. He has lived with the shame of his sin for all that time and feels a heavy obligation to make up for it any way he can.
The functioning of Eidolon's immortality is such that his body is impenetrable by objects or weapons and cannot be damaged in any way. He can absorb any sort of energy directed at him, contain it, and then release it at will. He feels no pain of the physical sort (because pain is a signal from the body that something is wrong, and nothing can go wrong with his body!), but endures a great deal of emotional turmoil from all that he has seen and experienced. In terms of strength and fighting ability, he's nothing special, but when you're immortal, you don't need to be. He's fond of saying that the only way to beat him is to bind and gag him, and then sit on him....forever.
Eidolon is also proficient in the performance of miracles -- if he were a bad guy, it would be called "sorcery" but like Lewis' Narnia series I have elected to allow for the practice of "white magic" on Hearthstone. Inevitably, because he has been through so many different ages and cultures, he has difficulties in interacting with the world as it changes around him. A good example of this can be seen on pages 26-28 of the first Shrike Team story. He also has mental powers much like the Vulcans of Star Trek, though there are limits in terms of what affect he can have on other persons, depending on what race they are from.
Though formally he will align himself with the Shrike Team, Eidolon will also "cross over" into the Range Patrol series.
Eidolon's future role is too critical to the overall plot to reveal...but you can see how we will use him in the future, in a scenario based in his past, in this team picture of the Karu'shi Av'nen (the Avengers of Av'nen) from Hearthstone's medieval period.
Eidolon is the leader, and he and his cohorts are on a crusade to render justice upon those who destroyed his people. A bit about his other four comrades....