Threadhopping with this character?: Absolutely. Threadjacking is pretty much always a-okay with me unless the subject matter or conversation is incredibly sensitive or emotionally charged. If in doubt. a PM never goes amiss.
Backtagging with this character?: PLEASE. It is never too late for you to jump in and it's never been too long for you to pick up an old, dropped thread.
Fourthwalling this character: He's an original character, so not sure how you would. People have tried "Angels don't exist", "God is a fairy tale" etc. with him before, and he has cheerfully countered that that's not possible, so, you know. Go for it? IDK.
Squicks, triggers, offensive subject matter: None that I can think of. PLEASE PLEASE let me know if I hit one of yours inadvertently.
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Hugging this character?: Oh certainly. Guri loves hugs!
Giving this character a kiss?: Sure - Guri is a platonic smoocher anyway. Anything more romantic or sexually charged is good for brain-breaking and general angel flail, always a good time.
Flirting with this character: It will go totally over his head. He has no flirt-dar whatsoever. Sorry, ladies and gents. But do it for the lolz if you like.
Fighting with this character: Oh, by all means! Verbal fights are fine with me, I have a good grip of IC =/= OOC and you won't hurt my feelings. And I love, love physical fight scenes, so go for it!
Injuring this character: Yes! Particularly in fight scenes. Guriel is not invincible and in terms of angel strength/endurance, he's near the bottom of the ladder. Hit me up by PM or AIM, I swear I don't bite, and injuries, if they're realistic and well-thought-out, are a-ok by me. I'm not going to let you maim him for kicks, but I'm perfectly willing to let him get his ass kicked.
Killing this character: Nooooooo. Sorry, guys, he is my bb.
Using telepathy/mind reading abilities on this character: It's been established through interactions with Lester's alien family that Guriel doesn't have a physical brain, per se -- his memory, thoughts, and emotions are all stored as spiritual energy like the rest of him (his body is a convenient shell, but it's no more part of him than a t-shirt is part of you). I'll leave it to your discretion if your telepath can pick up any thoughts from Guriel, or if he just reads as Not Human when they try. Please note that he definitely cannot be mind-controlled by traditional telepathic means and most likely won't notice that your character is trying.