To say Jiraiya was a lecher to his core wouldn't be entirely accurate.
"To the core" simply doesn't cover it; this was a man whose very cholesterol was saturated with lecherous genes. The trait had seeped into his bone marrow and vital organs. Long ago, he was a lecherous youth; now, he was a lecherous middle-aged man; and in the future, he would undoubtedly become a lecherous old fossil.
Lechery isn't a disease, but it can certainly be fatal... especially if you're caught in the act by someone eager to expose you.
Jiraiya was in the wrong, but his misfortune in this specific incident was a perfect storm of bad luck. Peeping was a minor transgression; his real mistake was crossing paths with this particular, "miraculous" pair of master and apprentice.
His "research" was exposed by the "good-deed-doing" Ui, leading to third-degree abrasions at the hands of an enraged mob.
Those injuries were then treated by Kushina, which meant those third-degree abrasions were likely to evolve into third-degree facial reconstruction.
In engineering terms, this is what you call a "two-stage turnkey service." One digs the hole, the other handles the burial. Without even realizing it, the two had executed a flawlessly coordinated maneuver.
But Ui had been careless. He had been in a post-nap daze, and being inside the safe walls of Konoha had caused him to relax. Roughly 80% of his IQ was offline; he had focused on being visually invisible but neglected to disguise his voice. He hadn't expected to run into Jiraiya again so soon, let alone that the Sage would have such a keen ear for the culprit's vocal signature.
Still, in the grand scheme of things, it was just a prank. Once Jiraiya realized Ui's identity, he couldn't actually do anything to the boy over a joke. As a Kage-level powerhouse and a village senior, he had at least that much magnanimity.
He was, however, profoundly annoyed.
Jiraiya had entertained the idea of giving Ui a "lecture" (likely involving some physical corrective measures), but the notion was ruthlessly shut down by Kushina. She immediately activated her protective skill: The Art of the Great Konoha Mentor.
"Jiraiya-sensei, honestly, I don't think Ui did anything wrong," Kushina declared. "Besides, at your age, why are you still doing these utterly pointless things every single day? Is it really that interesting?"
Yes, Jiraiya thought. Yes, Ui thought. Yes, was the word Minato wouldn't dare utter if his life depended on it.
Kushina didn't understand the male shinobi psyche. She assumed Minato and Ui were on her side, but in reality, the three of them shared a silent, spiritual bond—the collective perspective of the human male.
Nevertheless, they all nodded and acknowledged that Kushina's words were very educational. Being lectured by his junior was embarrassing for Jiraiya, to say the least.
Kushina glanced at Minato, reinforcing a long-held belief: even if Minato and Jiraiya shared a deep bond of master and apprentice, Minato had to maintain a strict, "restraining order" distance from Jiraiya's lifestyle choices.
Jiraiya shot Minato a look that clearly said: Help me out here, kid! Say something nice!
Minato gave a sheepish smile and looked away, pretending he hadn't seen the SOS. He was a smart man; he knew that defending Jiraiya now would only draw Kushina's indiscriminate fire. Regardless of how great a Hokage he might become, Minato was already well on his way to being a world-class practitioner of "Respecting One's Wife."
"Ahem..."
Ui finally spoke up. He was good at reading the room and realized it was time to give Jiraiya an out.
"Jiraiya-oji-san..."
Ui used a familiar, casual tone and the title "Uncle." Was there a flaw in the seniority logic? None at all.
"I'm really sorry about earlier. I truly didn't know the peeper was you. If I had known, I definitely wouldn't have exposed you... Based on our lineage, we're on the same team! Next time... next time I'll definitely cover for you."
"You're Ui, right?" Jiraiya sat up. "I've heard Minato describe you before. Based on those descriptions, I have ample reason to believe you knew exactly who I was when you started screaming."
Was Jiraiya, as a character, truly that suspicious?
Ui shot a side-eye at Minato. What kind of "indescribable" description had the Fourth-to-be given him? What kind of first impression had Jiraiya formed of him? It was hard to say.
"Anyway, let's just move past this. It's all within the realm of personal hobbies. We should seek common ground while reserving differences," Ui said, quickly trying to kill the topic.
Kushina didn't respond immediately. She absent-mindedly packed away the medical kit and stood up, moving away from Jiraiya to sit next to Minato.
"Ui, sit."
"Ah? Oh." Ui took the seat she had vacated. From there, he could smell the... "medication" on Jiraiya's face. At least it smelled like real medicine, suggesting Kushina hadn't actually been trying to poison the man.
"Ui, I heard you did great on the battlefield. You even almost blew up the Kazekage?" Kushina asked with genuine curiosity.
Blew up... That was a bit of an exaggeration.
Kushina knew the general gist of his performance; even stuck in the village, she had kept a close eye on any news regarding her only student. She cared deeply for his safety, but she wanted to hear the story from him.
"Well... an attack of that level could never kill a Kage. At most, the surprise element might have caused a few scratches. From what I saw of him a few days later, he seemed perfectly fine," Ui said with a cold sweat. Trying to take down a Kage-level ninja with bird eggs was a challenge of astronomical proportions.
"Actually, I didn't even know my summon had those offensive capabilities until that moment. If I had known, I could have planned it better and gotten better results... but I'd only signed the contract a few days prior. I didn't fully understand how those birds operated back then."