In Naruto, as is typical with Japanese manga, whenever a villain appears, it's usually part of some dramatic plot, shrouded in secrecy, which turns fans into conspiracy theorists. In fact, more often than not, these enemies usually have an emotional connection to the protagonist, whether they're faces from the past or affiliated with the hero's family in some way.
This was the case with the likes of Obito and Madara Uchiha, as well as the main villain of the series, Kaguya (the evil rabbit princess). Come Boruto, it seems that trend is continuing, because now Naruto, his son and their loved ones appear to be up against a familiar face from Naruto's teenage years -- his godfather, Jiraiya.
Jiraiya mentored Naruto, teaching him how to power-up into his Sage Mode and summon his kindred animals (in Naruto's case, frogs) in battle -- a key aspect of a ninja's evolution. As the series progressed, we learned Jiraiya was actually the man who taught Naruto's father, Minato. After Minato died, Jiraiya wanted to ensure his godson would harness the power of the Nine Tails Beast inside him as a force for good. More so, with the villainous Akatsuki hunting Naruto, looking to weaponize him, Jiraiya wanted to protect the young ninja from the terrorist group, who were led, ironically, by ninjas he also trained.
Sadly, the legendary ninja of Konoha would meet his death at the hands of the Akatsuki while trying to infiltrate their base. They outnumbered and overpowered him, impaling him with metal pipes before throwing his dead body to the bottom of a lake. It was one of the series' most heartbreaking deaths, but Jiraiya went out like a hero, even managing to send back crucial intelligence on how to stop the villains. Ultimately, his death served as a major turning point, helping Naruto mature into the Hokage (village leader).
Fast forward to Naruto and his son decades later in the Boruto manga, and a new mysterious terrorist cell, quite similar to the Akatsuki, has emerged: Kara. The person who stands out most is its head assassin, Koji Kashin, who knows a lot about Naruto's family. After engaging Boruto in a fight, he shows mercy, but is also impressed at his progress, which feels very similar to when Jiraiya first started coaching Naruto. Despite wearing a mask, Koji's long grey locks and his face resemble an older Jiraiya. What also fuels speculation that they're one in the same is the fact that Koji can summon battle toads, too. He can also use the Rasengan; a move only Jiraiya's stable of students could pull off.
Now, the Naruto franchise often depicts characters returning from the dead via several methods; some use forbidden ninja techniques (there's been cloning as well), but given his expertise, Jiraiya could have survived the Akatsuki onslaught using his chakra manipulation.
Even if it feels like we're shooting in the dark here based on physical appearance, Koji's disposition as an assassin with a code, one of honor and philosophy, paints him more as an antihero. Jiraiya loved to rebel against authority in the same manner. What seems to confirm they share the same identity is when Koji penetrates the chakra barrier protecting Konoha to investigate Naruto's new army. Only ninjas who mean the city well, have chakra associated with it or aren't foreign can enter, and Koji does so with ease, hinting the village is indeed a part of him.
Undoubtedly, this would be the biggest return in the franchise, especially after fans felt robbed by his murder. Many felt his relationship with Naruto was only just getting underway, with Jiraiya himself finally maturing from being a joker to the elite ninja we all heard about in stories of his legendary exploits as a Sannin.
Well, if Koji is really Jiraiya, it's going to be quite an emotional affair for Naruto. He's the only father the young man has ever known, and finding out he may now be a villain could change the Hokage and his family forever.