After a long silence, we're finally getting new Tales of Zestiria info! A new scan from Jump reveals a new character: Mikulio.
Mikulio ( ミクリオ, mikurio, romanization not yet official) - designed by Mutsumi Inomata, voiced by Ryouta Ohsaka.
"Slay is that kind of guy. Even as his childhood friend, it's agitating to watch him."
Mikulio is Slay's/Thray's best friend who is in fact, one of the Divine (please read here for information about the Divine and whatnot). He's 165cm tall and his weapon is a long staff. He is Slay's childhood friend and also his rival, and is calm and composed guy as compared to Slay. Since Mikulio is one of the Divine, he is actually invisible to normal people, and only Slay can see him. His artes seem to be focused on water.
The scan also mentions that Slay actually lives with the Divine, as well?
Scan from @kazu4281.