I love the Bleach and HXH crossover!! It’s such a unique idea I never thought of! How far back in the timeline is Toshiro in? Does any of the current Zoldyk family members with white hair remember who he is or do they just start to forget him as they grow older? (Not Killua obviously)
Toushiro is literally the younger brother of the First Zoldyck. He's basically become a Zoldyck legend at this point - the guardian spirit that only proper heirs can see (aka the white hair = reiryoku).
Killua, however, is the first kid in a few centuries at least to be able to clearly see Hitsugaya - which is why Toushiro becomes so protective of him. Silva, Zeno, missing person (Zzigg?) and Maha can all sense Toushiro and see a fuzzy blur, but none of them have enough reiryoku to become a shinigami, or anything close. They still know when he's around because he does ghost things, but they don't really know him at all.
But Killua DOES have the reiryoku necessaryto see Hitsu. And Toushiro knows that. So he hangs around when he can, purifying the Hollows that appear when Killua kills someone, so his great x 100 nephew isn't hurt. But he's not there all the time, so he missed the needle being put in and etc.
Thus, when he notices the needle (he can see/sense Nen - being dead usually means you can see life energy yanno - but can't use it since he's not alive) he kinda. Goes apeshit.
Illumi is only alive because he's related to Hitsu, lmao.
Oh, and IchiHitsu is an established part of this. It's set WAY post Bleach canon, so Ichigo is legit dead now and Hitsu is full grown, and they're lowkey awkward-dating.
And now I'm rambling- djdndnebdnxjjdej