In the climax of chapter 22 of Boruto Manga, it was shown that Kashin Koji is able to perform toad summoning. The previous known users of Toad summoning are Jiraiya, Minato Namikaze, Naruto Uzumaki and Konohamaru Sarutobi. Since all these users of toad summoning are related, and now since Kashin is also a toad summoning technique user, he should be connected to Jiraiya somehow.
Before getting to the part how Kashin Koji maybe connected to Jiraiya let me give a brief introduction about him. Jiraiya was one of the three legendary Sannin of Konohagakure, along with Orochimaru, and Tsunade Senju. While Tsunade Senju and Orochimaru are still alive, Jiraiya by the looks of it, died during the Pain Arc in Naruto Anime. Jiraiya was the first known user of Toad summoning technique. He was a very powerful character, and the fact that he took down 3 of 6 Pain bodies without any prior information means a lot really. Jiraiya was truly a legend.