One of the twelve Zoalords. He is the largest of them in human form, and has a cracked-looking and (apparently) rock-like body. So far he has only rarely appeared, never speaking, thus not much is known about him. During the assault on the Relic, at Mount Minakami, he was seen spewing fire.
The name Galenos appears to have no obvious connotations: the only famous historical Galenus was a doctor of medicine. Tuatha dé, on the other hand, is an Irish term which translates as "people of the god (insert name)", as in "Tuatha dé Danann"- the people of the goddess Danu.