A lot of characters are "immortal" or at least doesn't age. Most of them lived since the First War and seems to be strong as hell compared to the current generation.

Lilianna is a formless being who lived since the beginning of time, abaldum is an ancient being who lived for thousands of years. So are Chaugnar, Lumburr and the likes.

As about people who participated in the First War, there're the drama about Volkath x Marja and Illumia. Tel'annas finished Volkath and went to sleep. Arthur is a norman so he probably died. Sephera was also there.

Richter and his demon hunter friends also sided with human during the war, note that the first Demon Hunters appeared 800 years ago prior to Valhein's generation.

Errol lived for centuries until Hayate found him. He once fought along Darcy, who were revered as a God back then.