He is one of Apollo's slaves who knows what he wants and pursues the god himself. He gives Apollo an insight into Alexios' and Galen's relationship that he would have otherwise not understood.
Here's an unedited excerpt from Alexios' Fate featuring Halys.
Apollo awoke before the dawn. He needed to be at his personal shrine to receive the prayers of his devotes as they performed their monthly purification rites.
He reached through the sumptuous blankets and furs for Halys only to find the slave missing from his bed. He sat up and ran a hand through his hair. A rapid sweep of the dimly lit chamber failed to yield the demigod. Bronze lamps on either side of the bed did little to banish the stygian shadows in the corners.
Just as he considered summoning another slave, Halys entered the room. The aroma of porridge laced with nuts and honey filled the air. Apricots and figs filled a small bowel and a kylix of ambrosia balanced beside them on the tray.
Halys nudged the door closed with his hip and Apollo recalled all the skills those supple hips demonstrated the night before.
"I hoped to return before you rose."
"I just now awoke. What do you have for me?"
Halys broke into a pleased smile as he crossed the room. He placed the tray on a small table near the bed and offered Apollo the ambrosia.
When Apollo finished breaking his fast, Halys brought a woolen robe trimmed in red and gold scrollwork. The slave draped the folds over Apollo's shoulders and settled the sacred laurel wreath like a halo on his head. Halys knelt at his feet and laced his golden sandals, dark fingers straying over his calves and ankles.
With an indulgent smile, Apollo lightly cuffed his slave. The creature grew more and more forward. Unless something changed, Halys was dangerously close to becoming a spoiled pet.
"I will return in a while."
"Yes, master. Do you wish anything upon your return?" That mischievous smile again.
"Do you think of nothing else?"
"Not while I'm in your presence."
Shaking his head at the daring demigod, Apollo strode from the room.