Whether he had a brother, or a wife or a child, or who his parents were is quite mysterious. Kashin Koji appears to have white textured hair; same as Jiraiya. So, in my opinion, Kashin Koji could be his brother, or son. Not only they have similar hair, but they have similar body physique as well. Or there could be another scenario in which Kashin Koji might be one of his another stronger students.
I’m afraid that Kashin Koji might be about to kick Team 7’s ass. I don’t see Team 7 winning against Kashin Koji. I think that someone will have to come and rescue the Team 7. Their team captain was disappointingly defeated while they were battling Ao. The only other way in which I see Team 7 not dying is that Kashin Koji allowing them to live on, and showing mercy. This is the point where I feel that Koji might reveal what his connection with Jiraiya actually is.