He is a police detective in a large city whose government was corrupted by at least two villains until recently. Ishida, however is a steadfastly honest officer with a deep commitment to the cause of justice, regardless of who the criminals are. He is quiet in his manner, but highly observant and astute in his reasoning. Those faculties are invaluable in difficult cases like murder where intensive examinations of dead bodies like an autopsy are taboo. In addition, when force is called for, he has formidable fighting skills to call upon using his standard police weapon, the jutte, although he is allowed to also carry a katana.
Ishida first appeared in "The Hairpin Murders" in which the detective, with the help of Miyamoto Usagi, investigates a strange series of murders where the weapon is a hairpin stabbed in the neck. Although the character was intended only as a one shot, he soon proved unexpectedly popular and series creator Stan Sakai has used him in numerous other stories.
In "After the Rat," Inspector Ishida appears in his own story in which he is after a Robin Hood-like character named Nezumi, who steals from wealthy merchants, and buys the favour of the people by donating a portion of his gains to the poor. After Nezumi is accused of murder, Ishida's deductive skills tell him that the thief is no murderer, and despite his resentment, Ishida will not tolerate the crook being charged with a crime he didn't commit.
In the same story, Ishida has a run in with the series' most notorious criminal, Kitsune, but is not fooled by her appearance as a street performer; although he knows the truth, with no proof he subtly demands that she leave, disguised as a compliment. For her part, Kitsune realizes that this police officer is not to be trifled with and decides to acquiesce.
A new story featuring Inspector Ishida is scheduled for the near future:
"UYD Member "Maka" gave Stan a great idea for an Inspector Ishida Mystery, a Hana-fuda crime for Inspector Ishida to solve. "The murderer leaves hanafuda cards in his victims clothes as a clue to why the crime was committed." Stan thought it was a great idea and, with Maka's permission, will be using the story in a future issue of UY!" 
In "The Hidden" story arc, Ishida was involved in a murder case of two Samurai, whom one of them was a member of the outlawed Kirishitan (Japanese term for Christian) religion. Despite Ishida sharing the skeptical views on Christianity as Usagi, especially in regards to the afterlife, he generally turns a blind eye on suspected Kirishitan, especially Hama. Later on, his case led to clashes with Shogunate agents who seek to seize the book while Ishida seeks to use it as a means to bring justice to murderers. As the two groups fought bitterly, Hama later grabbed the bible and plunged himself into the fire--even as he was stalled by Ishida--as he found no means to escape from the Shogunate agents. With the book seemingly destroyed with Hama's matyrdom, the Shogunate agents called off their mission and left the scene. In the end, it was revealed that Ishida himself was a member of a secret Kirishitan group and managed to recover the bible from Hama during his struggle.
Usagi Yojimbo Vol. 3 No. 30 (Tameshigiri)
Usagi Yojimbo Vol. 3 No. 77 (After the Rat)