Detective who works under Detective Gotou and who starts out loving the supernatural but after experiencing it first hand is scared of the supernatural. However, he still remains very interested in supernatural phenomena, at times suggesting the causes behind events are because of it.
Although Gotou doesn't think too highly of Ishii, Miyawaka and Yakumo thinks highly of Ishii. Throughout the series, Ishii slowly changes into a person more sure and confident of his abilities, even later starting to voice his opinions against Gotou. He admires and respects Gotou, wanting to be a detective like Gotou. He has a crush on Haruka.
In volume 5 of the light novel, Gotou got kidnapped. Ishii was able to eventually figure out the case and where Gotou was hidden and saved him. Although he was led into saving Gotou by the real culprit, it was the first time Ishii was on his own and was able to figure things out.
Shinrei Tantei Yakumo: Another Files
In the first volume more of Ishii's past was revealed. He didn't have much friends during school, he closed himself off from others. He wrote a manga and had wanted to go professional one day but his father did not allow him and so Ishii ended up being a detective.
He was bullied during high school by Kosaka Yukari and Mochizuki Toshiki. Ishii was called "glasses monkey" by them. Due to the bullying he came to hate them and the thought of wanting to kill them came into his mind often. Because of this Ishii believed that he was the one who killed Kosaka Yukari (by pushing her down a shrine's stone steps), though of course it was later proven to be false.
At the end of the volume it was revealed Toshiki had in fact wanted to be Ishii's friend at first. He approached Ishii asking about the manga Ishii wrote. However Ishii denied it and pushed him away. It was after that that Ishii was bullied.
