WARNING: The following contains spoilers for Boruto: Naruto Next Generations Episode 184, "Puppets," now streaming on Crunchyroll.
One of the most heartbreaking moments in Naruto came when the legendary Sannin Jiraiya died at the hands of the Akatsuki. Jiraiya was Naruto's godfather and closest link to his parents, Minato and Kushina, being a father-figure for him in ways Kakashi and Iruka never could. Boruto offers a similar profound, heartbreaking death involving a father-figure to the hero. When you think about it, Mugino is like the modern version of Jiraiya.
Jiraiya taught Naruto to dig deep and become a warrior his dad would be proud of. The Hokage, Tsunade, Naruto's teachers and peers were all shocked the ninja was evolving this quickly. Jiraiya was a jokester, but he had a profound effect on his pupils, not just Naruto but also Minato, Nagato, Yahiko, Konan and others. Jiraiya eventually died in a sacrifice that helped Konoha win the war and change for the better.
Mugino has the same effect on Boruto, despite initially being at odds with the youngster. While the manga didn't have this much depth to him, the anime does Mugino justice, making him out to be a truly selfless shinobi in his duty to Konoha. He's not a roamer like Jiraiya, but they're both dedicated to the village. Boruto becomes closer with Mugino when he hears about Mugino's story of redemption after being a child assassin. When Mugino expands on the stories of the Third Hokage, Hiruzen, it feels like when Jiraiya told Naruto old war stories -- and these have big impacts on both generations of Uzumaki warriors.
These great mentors believed in something other than battles and wars. Jiraiya taught Naruto to really see the outside world, not as abstract villages and nations, but as people worth trying to understand, whether they're friends or foes. Mugino does the same, going even further in teaching Boruto about compassion and understanding people who need charity. Mugino is a social justice activist who fights for the poor. All of this goes beyond what Sasuke, Konohamaru and even Naruto have taught Boruto in their sessions.
Like Jiraiya did for Naruto, Mugino digs deeper for Boruto. Mugino's death evokes so much heartache for Boruto as a result. It's ironic that both Jiraiya and Mugino die as heroes but their enemies live on to fight, but both men would do it all over again to save innocent. Just like Naruto became a better man because of Jiraiya, Boruto is all the better because Mugino encouraged him to be a critical thinker.