Athes is demon from the alternate Cythera world. In his reality, mages pursued learning in how to conjure demons (since demons are beings of magic). This led to more and more manipulation and creation of demons. In the Dark Mirror TS it's learned that Saltwax and other forces created large demon armies of varrying strengths. These demons would not be considered in the same class as Athes. Athes is a demon born of naturally occuring magical activity (i.e. hellspawn). He considers the new waves of demons as abominations, slaves, and needing of extermination.
Athes was picked up by Firecat after Athes slaughtered a demon troupe sent to kill him. Athes knows that Firecat was involved in sending the troupes, and also knows that he works (or at least used to) work under Saltwax, one of the main users of magic-spawned demons. Firecat gained Athes' help through offering a way to destroy all magic-spawned demons, as well as their production. In the DM TS Athes was instrumental in burrying the demon troupes that had been sent to the original Cythera, causing a cave in the killed and burried most of the small army. He then hunted down those that escaped and then caught up with Sage after doing it again in Catamarca.
Even though Athes works under Firecat, it doesn't mean he likes how Firecat opperates (or Firecat himself). The demon would prefer a straight forward approach of facing the enemy, rather than the covert ways of attack Firecat usually sets up. Athes and Firecat's relationship contains a lot of tension and butting of heads. Athes knows that Firecat is stronger than him, even though sometimes his pride will lead the demon to challenge him anyways. Firecat has never taken direct control of Athes, nor could he given Athes' strong will against him.
Athes main motivation is in restoring balance to the world through violence. He does not tend to see solutions to problems as changing the bend of the river, rather just damming the river all together (ending it). His main goal is to restore demons back to their natural order. His secondary goals were gained after meeting Firecat and learning of the elemental disbalance in the alternate Cythera. In his perfect vision he would not have Normal Cythera, he would have a Cythera where his kind are more plentiful. A place where strength and truth ruled supreme. Athes will not hesitate to tell the truth to those he thinks have earned it. He only lied to the group in the beginning of Out of Reflection because he was ordered to. He is under orders not to reveal Sage's true name however.
Though Athes has these ideals at heart, it does not mean he does not get distracted from them. He takes humor in others being foolish. He does not care much for jokes involving words or sentences. In fact he would rather act than talk all together, never learning far into making words sound pretty. He also has a love for the sport of killing, and the skill involved; in this way, he can gain some level of respect for people.
Athes' has a great discription in my first post of his, but here are the cliff notes.
Dark red skin. Two three-quarter-foot long purple horns coming out of his head, along with a bit of dark greyish hair. A white, twisted cloth band around his forhead. Dark, black eyes, no whites. Fanged teeth. Very muscular build, but average male height. Hard to tell his age because of his demon's face. No shirt, brown loin cloth going to his knees. No shoes. Clawed hands and feet. (Pretty much look at a demon sprite from the game). Athes' voice is also unique in that it is low in tone, and crackly (not in a puberty kind of way, think of rice crispies). His speech pattern is as few words as possible, and a direct manner of saying them. Athes typically smells of ash and smoke. Athes is also exceptionally hot to the touch. Athes does not like the cold.
Being a demon of natural blood, Athes posesses a wide range of magical abilites: fireballs, earthquakes, lightning, fear spells, etc. In addition to this Athes is good in fighting with his hands, magic, as well as his blade. Athes will not hesitate to take a cut to get the upperhand in a fight, as he has a extremely high tollerance for pain. Athes weapon is a double sided obsidian swallow (something like this). He is very proficient with it. If the glass ever breaks or becomes dull, he'll use fire magic to make a new blade or to fix one. It is the weapon of his tribe of demons.
If more info is needed let me know.