Hitsugaya reported the death of a Shinigami in Naruki City two months ago, and Isshin expressed knowledge of the incident, fully aware the Gotei 13 was already investigating the incident.
When Hitsugaya further reported two more individuals had died within the past month alone, Isshin, concerned with the disappearances, decided to investigate them alone, knowing Rangiku and Hitsugaya were not strong enough to assist him.
At some point, he attempted to learn Saigo no Getsuga Tenshō by facing his Zanpakutō's spirit, Engetsu.
Whether he succeeded or was unable to learn this technique is unknown.
He has the appearance of a man in forties, as noted by his daughter, Karin.