Kosaburo Katsura

Kosaburo Katsura is a private investigator in Digimon World Data Squad, a side-story to the main Digimon Data Squad universe. He is a civillian with a Digimon partner that acts outside the rules of DATS, making him something of a wild card in the story. His partner is a Biyomon who thinks very highly of Kosaburo's skills as a 'master detective'. It's implied that Kosaburo has interfered in some of DATS' (particularly Yoshino's) missions in the past when his clients' requests are at odds with DATS' assigned missions. It is unknown how Kosaburo accesses the Digital World without DATS' Digital Dive technology.

Description
In Digimon World Data Squad, Kosaburo has taken on a client who has requested that he collect the Code Keys of the Seven Great Demon Lords as they appear in the Digital World. When the DATS team saves a young boy who had become a part of Belphemon's data, Kosaburo's Biyomon swoops in and snatches the Code Key of Sloth from the team before they had a chance to analyze what they had found. Kosaburo appears and taunts the team a bit, thanking them for doing his job for him before escaping with the Code Key. Yoshino recognizes him and laments how he has interfered in her missions before. The DATS team decide not to follow him and concentrate on helping the young boy.

Kosaburo appears again when the DATS team save a young girl who gotten trapped in the Digital World and had become a part of Barbamon's data. Biyomon again swoops in and snatches the Code Key of Greed from the team. Yoshino demands to know what he's up to and he replies by teasing her about her scary face and how it sabotages her relationships, indicating that he and Yoshino know eachother pretty well. Inevitably, he escapes again.

When the DATS team rescue the next child from Lilithmon, Thomas and Gaomon prevent Biyomon's predictable attempt to capture the Code Key of Lust. Kosaburo, determined not to fail his client's request, produces a Digivice iC and evolves Biyomon to Birdramon with his own Digisoul charge. They attempt to take on the DATS team but with Renamon's help, Kosaburo is defeated and they are forced to flee.

Unable to collect the Code Key, Kosaburo contacts his client and informs him of the situation, noting that the client had gone to great trouble to keep his identity secret. The client accepts that DATS has gotten the Code Key of Lust and tells Kosaburo to transfer the other two Code Keys' data to him in exchange for wiring Kosaburo's payment. The client states that this marks the end of their business contract, and moments after the transfer is complete, Kosaburo and Biyomon are attacked by one of the Demon Lord digimon, Creepymon.

Their fate is unknown until the DATS team collects the fifth Code Key of Envy. Kosaburo and Biyomon suddenly appear, somewhat injured and having barely escaped from the Demon Lord, to pursue their traitorous unknown client. It turns out that Kosaburo had placed a tracking program on the Code Keys that he sent to his client, just in case such a situation should arise. Kosaburo's device leads them directly to Tsukasa Kagura, one of DATS' own technicians who had been leading the missions to collect the Code Keys in the first place. The DATS team is skeptical of Kosaburo's words at first, wanting to trust their own teammate, but with Kosaburo's tracking device blinking obvious from Kagura's pocket, the DATS team have to ask him to show its contents.

Trapped by Kosaburo's evidence, Kagura gives up his ruse, snatching the Code Key of Envy from the team and attacking them with some DemiDevimon and Devimon to make his escape. Heading back to the real world, the DATS team seems willing to overlook Kosaburo's past actions thanks to how he exposed Kagura's betrayal.

But Kosaburo is insistent on helping the team stop Kagura. At first, the DATS are wary of letting a civillian get involved, but Kosaburo makes an impassioned speech about wanting to get payback on Creepymon, which sways Marcus to argue on his behalf. The Commander agrees to the suggestion and Kosaburo travels into the Digital World with DATS on their next missions to stop Kagura's plans. Yoshino makes a point of teasing him by saying that if he's so eager to help, she'll accept him, but only as a lowly grunt whom she'll work to the bone.

From that point on, Kosaburo occasionally lends a private eye's perspective to tracking Kagura and solving the mysteries of his plans, as well as helping Biyomon to fight in the numerous battles to stop the ressurrection of the Great Demon Lord Digimon, Lucemon.

After defeating Kagura, it is presumed that Kosaburo and Biyomon return to their private investigator work, provided that they don't interfere with DATS missions anymore.