Illumi Zoldyck, a renowned assassin, a notoriously twisted little-brother-obsessed brother, and Killua's number-one fan.
As he sees it, although his little brother at home is occasionally rebellious, he still loves his big brother, all in all.
Firm in this belief, Illumi secretly followed Killua and Alluka, crossing one mountain after another and one river after another. Each time Killua let his guard down, Illumi would suddenly appear, sincerely and kindly trying to get his little brother back in his arms.
Illumi: I'm being so enthusiastic. Kiki must be really touched, right?
Killua: Not even a little, thanks.
The rebellious little brother trampled all over his big brother's good intentions.
Killua: Let's go somewhere Illumi can't find us, Nanika!
Illumi: !
His little brother had vanished before his eyes.
He immediately found a Nen user with the ability to locate people, only to learn that Killua had gone to another world.
Illumi: ...
This won't do. Even a rebellious phase needs limits, Kiki.
I've decided. Once I find Kiki this time, I'll give him a thorough lesson in love!
However, crossing the barriers between worlds is no simple matter.
So, across all sorts of worlds...
Illumi (with damaged memories): I remember I came to find my little brother... Where is he?
Many years later, little brothers (quite a lot of them) were finally born.
Illumi (with damaged memories): Don't worry, little brother. This time, Big Brother will give you even more love and nip your future rebellious streak in the bud!
Uchiha Itachi (during the Akatsuki period): It's good that the tragedy of the Uchiha clan was resolved like this... And I really wasn't going through a rebellious phase before, Illumi. Could you stop staring at me?
Uchiha Sasuke (during the Boruto era): Sorry, Itachi, but seeing you get put in your place by this new big brother is kind of satisfying... I just wish the new big brother wouldn't keep taking issue with Naruto's voice...
Tsugikuni Michikatsu (after the finale): ...I... am no longer... the eldest son of the Tsugikuni family...? That's wonderful... Yoriichi... thank you... Brother...
Tsugikuni Yoriichi (the only original): Not love my brother? How could I not? (munch munch) Big Brother is so good to us, always running around taking care of everything, and even playing with us (munch munch). We're his little brothers, how could we let him worry? (munch munch)
Killua (making a glorious escape with Alluka): What kind of ridiculous rose-colored glasses are you wearing?! Get a grip! If Illumi had even a tiny bit of what a big brother should be like, he wouldn't be completely lacking everything a big brother should be like!
Theme: A little trick for resolving family conflicts
