It was because of Jiraiya that Minato Namikaze and Naruto Uzumaki learnt the Summoning technique, and later on now Konohamaru Sarutobi can also use it. Konohamaru Sarutobi might have learnt it from Naruto Uzumaki. Jiraiya was a very powerful character in NarutoVerse. Jiraiya was also the user of the Sage Mode of Mount Myōboku.
Now, coming to how Kashin Koji and Jiraiya’s connection. As mentioned earlier, in some way or other all the users of the toad summoning technique are related to Jiraiya. Now, it means the Kashin Koji has to be someone that Jiraiya was in close contact with. There are some theories suggesting that Kashin Koji might be Jiraiya himself, but I do not think so, as Kashin Koji does not appear that old to be of Jiraiya’s age. What is interesting is that in Naruto Anime Jiraiya’s family was never discussed.