Disclaimer: The characters from Artemis Fowl belong to Eoin Colfer and his publishers. 
Author's Note:The ideas/characters from Ancient Greek myth are just that, and so belong to 'Culture', inspired by Biz who named the Artemis/Butler ship 'LE Ceberus and Hunter'. And, believe it or not, this was the product of listening to Aqua.

He never slept.  He only dreamed.  He wanted, craved... knew better.

He lived, ever vigilant, as Ceberus always is, always was, always would be.  He guarded hell.  And all its occupants, all its eccentricities.  He guarded Hades from harm, sitting back on his heels and asking for a stomach rub when the Master passed.  He didn't concern himself with right and wrong, because all to do with Hades was good.  All of it meant he could serve, be beside him, make him happy.  That was all that mattered.  Nothing mattered as much as Hades.  Nothing could.  Why would it?  Hades was the world, and his Underworld was his playground.  Ceberus had to protect Hades' playground, because then Hades would be happy.  And only when Hades was happy could Ceberus be happy.

As Hades went around his business, Ceberus went around his own.  He protected, he protected the Underworld from everything that might cause his master harm, everything that the Master perhaps would not want or enjoy.  He found satisfaction in his occupation.

He growled at a stray traveler who was trying to gain his master's favour.  The Master belonged only to Ceberus, not to no one else.  The Master loved him; and he loved the Master.  He loved the Master completely.  The Master was his life, all his life, was all his meaning.

The Sphinx was jealous, envy streaking from her in waves of darkness and conflict and pain.  She was a fury, dancing in her anger.  Ceberus barely noticed, and then only enough to tell her to be careful around the Master because he did not like being disturbed while he was working.

She fled from the cave, fled to the surface, the light.

Ceberus almost felt guilt, but then he remembered the Master's pleasure and approval and everything was right, everything was perfect.

The Sphinx cried when she was Above, dancing her anger in the light of Apollo.  But in light anger softens, disappears until she wishes to try again.  They are both guards, her and her brother of the Underworld.  She knows him, trusts him, understands him... Yet not.  She lives, he does not.

She cries, and then leaves.  Not wanting to ever come back.

Ceberus is content, since Hades let him into the Underworld that day and patted his right head.

Everyone knows that Ceberus can be tamed by music, though no one considered him being tamed because he wanted to be.

When next the Master passed Ceberus whined for a stomach rub, for recognition of his deeds and position.  Hades ignored him, stepping over his prone body.  He had better things to do.

But Ceberus didn't notice the dismissal, because he had been trained against emotion.  He never had emotion.

Ceberus cried, his whines almost a song in itself.