Now there's a lot of mons Saix could be. He could be a Midnight Lycanroc, he could be any other mon associated with the moon but I chose an Umbreon for several reasons: first, his ears would be dropped by a birth defect which resembles his hair bangs. Second, since he values his (dead) bond with Axel so much that may have triggered his evolution since we know it requires high friendship. And third, to be a shiny to reflect his blue hair and to be special due to his connection to Kingdom Hearts.
And fourth, Umbreons are simply dope and so he is.