Like Jiraiya, Minato and Naruto, Kashin Koji uses the Rasengan and can even summon toads. Thus, drawing a connection between Jiraiya, Minato and Naruto Uzumaki.
It is still unclear how he knows how to use such techniques and we won’t be surprised if he knows how to use Sage Mode of Mount Myōboku as well.
Now, coming to his identity. I do think that he is certainly connected to Jiraiya and I have a theory that suggests that he is Jiraiya’s son. Well, let’s try to fit all this in a perfect imaginary timeline. First of all Kashin Koji says that something about Konohagakure always brings him back and thus we know that there is some sort of connection between Kashin Koji and Konohagakure. I don’t know whether you have noticed it or not, we have seen it on more than just one occasion that he’s trying to save the Konohagakure shinobi in the manga.