"Kacchan!" The Sand shinobi - the one-tail jinchuuriki - calls out happily upon spotting him, which instantly sets off all kinds of alarm bells, and not just because an ally-in-name-only has recognized his baby cousin by sight alone.
There are only three people in their entire village, Clan included, who look so delighted when they bump into Shisui's gremlin of a relative, and they are: one, Katsuki's mother (who is terrifying in her own right); two, his brother (who is only slightly less terrifying and only because he answers to Lady Uchiha); and three, Uzumaki Naruto (who could probably make a cold-blooded killer repent and change his ways if only given the chance to talk to them, which is in some ways even more terrifying than the other two!).
Shisui loves the little hellion to literal death, but his first reaction when seeing Katsuki nowadays is almost always dread followed swiftly by either remorse (if he is spotted and was the intended target) or relief (if he isn't). He only allows himself to feel joy once he's gotten a proper read on the situation and is sure to come out the other end un-singed.
Okay, yeah, he thinks it's absolutely hilarious inflicting his cousin on other unsuspecting persons and watching them attempt to deal with a pint-sized kid with the mouth of a fisherman and the temper of a rampaging bull, but that's from a safe distance!
No normal person looks at Uchiha Katsuki, self-proclaimed King Explodo Murder, and smiles so genuinely, much less intentionally beckons him over.
At least Katsuki looks as bewildered as Shisui (after the initial rage at being called such a cutesy nickname) before indignation takes over, which makes no sense at all.
"Fucking Deku," he spits, face contorted in an amusing snarl/grimace as he reluctantly stomps right up to the still smiling boy and points aggressively. "What the fuck! What, stalking me before wasn't good enough for you?! You had to fucking follow me here, too?!"
Subaku no Izuku is unruffled. Well, no, actually he begins to tear up, but his next words prove that it is not out of fear. "I'm so glad to see you, Kacchan! I thought I was alone!"
He sniffles, and Katsuki looks so offended, Shisui wouldn't be surprised if he decided to throw down right here and now, shaky alliance between their villages be damned.
"We are not friends!" Katsuki emphasizes, incensed. "Don't get in my way, shitty Deku! I'm gonna be the strongest!"
"I missed you so much," Izuku wails, crying now, regarding Katsuki with no small amount of fondness and disregarding absolutely everything he's saying and doing. "I can't believe you were in another village this whole time."
"I can't believe you're still such a fucking crybaby," Katsuki sneers. "You're here for the chuunin exams, aren't you? You're gonna make people think Suna's fucking weak."
"Kacchan doesn't have to worry about me," Izuku says the wildest thing Shisui's ever heard, with an adorably sweet smile. He has no fucking idea what is going on. "I wouldn't have signed up for the exams if I wasn't confident in my abilities."
His eyes burn with determination, and strangely enough, it is this that finally gets through to Katsuki, who bares his teeth and demands, "You'd better make it to the finals, so I can finally beat your ass fair and square."
"I'm excited to see how you've adapted, Kacchan, but I'm not planning on losing," Izuku rebuts, not shying away like most of the poor fools faced with Katsuki's full attention.
"…How does Katsuki know Suna's jinchuuriki?" Shisui asks the universe in general.
Alas, the cosmos remains, as ever, silent, and it is instead Sakura who answers him, posing a very important question: "Are you sure you want to know, Shisui-san?"
Shisui considers that. Whatever the hell's going on between Katsuki and Izuku seems unnecessarily complicated and full of myriad unspoken and unresolved issues, for all that they seem to be on good terms.
Is that a sealing scroll he wants to unpack?
"No," Shisui says, at peace with himself and the world. "No, I don't think I do."
Next to him, Sakura seems to have come to the same conclusion if the serene, Buddha-like expression on her face is anything to go off of. "A wise decision, Shisui-san."