Xeross
Xeross is in fact a demon whose master is the Greater Beast (Look at characters list). Demons can take any shape and Xeross takes the shape of a human male. He first met Lina in a forest and helped Lina. (At the time Garv (characters list) was rebelling against the other demons and was trying to use Lina's magical power to rule the world. The rest of the demons did not want this to happen as demons live on humans pain and agony and suffering). Going back to Xeross, he is sometimes an enemy of Lina sometimes a friend and sometimes they travel together. Lina knows he is a demon but does nothing about it as he does help them.
Xeross is one of the strongest demons apart from the 5 leaders and the Lord of Mightmare (characters list). He can teleport and his magic powers are not totally seen yet: probably as strong as Lina.
Just one thing left. He always uses the phrase "That is a s-e-c-r-e-t".