Teres

I've been thinking of him as a guardian fey or protective spirit, so it's honestly not a terrible guess. (Yr in particular has a lot of malevolent fey vibes, and the others aren't that much better.)

And given the quality of Douglass' world-building, I doubt Charonites are ever going to stop being an option. (I'm honestly a little surprised she bothered assigning the Sentinels a species instead of just leaving them as Sentinels.)