I noticed the resemblance between Hiruzen, Asuma and Kashin Koji and noticed they all look almost the exact same. Kashin Koji looks exactly like Asuma, but he has Hiruzen's eyes and the stripes young Hiruzen had. He isn't as tall as Asuma though. He looks around Konohamaru's height.
He was said to be in Anbu and directly under the Third's command. Maybe that's why he was able to infiltrate the leaf so easily. He even knew it's secret password that only a few Jonin and anbu might know and him mentioning about him being linked to the leaf could also mean something. Also, he specifically made a note on Konohamaru when they met each other, when Konohamaru isn't even that well known in the leaf. Overall, he seems way too much powerful compared to Asuma though. It seems he inherited more of Hiruzen's powers than Asuma.
What my theory is that Kashin Koji was in the same team as Minato and the other guy in team Jiraiya, which is why he knows Rasengan and Toad summoning. I mean, just take a look at the photo of team Jiraiya. The guy in the far left is wearing the same outfit as young Hiruzen ( On second, it's possible that Hiruzen asked Jiraiya to train Kashin Koji ). Also, since Sarutobi clan members are proficient in fire release, it could explain why he has a powerful fire release like True fire of Samadhi. The guy is literally the son of Hiruzen Sarutobi. Kashin Koji is most likely around 10 years older than Asuma, as he was in Minato's generation, with him being around 28 years older than Konohamaru. That would make Kashin Koji around 57 years old now.