Given Dezel's current situation of being a former loner who's trying to not be so much of a loner anymore (and being awkward about it a lot of the time), that could make for some interesting CR. (Though there's a fair deal about Dezel that is spoilery, so do let me know if that's an issue for you!)

My assumption is that Dezel won't be visible to Killua, though, but you can correct me if I'm wrong. Typically, seraphim can't be seen or heard by humans, not unless said humans have a high level of spiritual insight/resonance. Here at Emp, sharing an alignment with a seraph will automatically fix that issue, but Dezel is with Elios, SO. Yeah. Anyone can communicate with seraphim using telepathy, though, so there's been a lot of that going on with the mostly invisible part of the ToZ cast, here.