2nd lieutenant Koito is a member of the 7th division of the Imperial Japanese Army. He met 1st lieutenant Tsurumi as a child, who encouraged him to join the army after staging Koito's kidnapping and subsequent rescue. Koito admired Tsurumi as an ideal hero from this point, carrying around a photo of him and lamenting the fact that Koito couldn't have been by his side earlier.
Koito is willing to do just about anything to prove his usefulness to Lt. Tsurumi, and he's so nervious around him that Koito can't even speak to him normally, much to the annoyance of Koito's sergeant, Tsukishima.
Basically, Koito is introduced as an over-the-top and somewhat comedic character that is still a highly competent antagonist for Sugimoto and co. I remember I caught up with the Golden Kamuy manga just after the Abashiri prison arc was coming to an end, so Koito hadn't had much focus yet besides his introductory scene. It was exciting when Koito ended up traveling with the main party for the Karafuto arc.
To be honest, I remember initially thinking Koito was an interesting character because of his attractive appearance (he's supposed to be a pretty-boy in universe, so...) and obsession towards Tsurumi. The image of him looking sadly and pathetically at his Tsurumi bromide while lying defeated in a tree canopy after failing to stop Sugimoto's escape on the blimp immediately took my interest. There's something about an obsessed subordinate character that I usually tend to like. However, unlike most of those characters, Koito's main development is his struggle to eventually break out of this role and become a leader himself, which I think is a very compelling narrative. Out of Tsurumi's 'harem', Koito is the only one to achieve this despite appearing the most outwardly obsessed with him. Usami is happy to die as Tsurumi's tool, Tsukishima just decides to follow Koito instead, and Ogata, well... You know.
You can definitely argue that Koito's ending isn't a "good" one as he still stays within the Imperialist military structure and never admonishes Tsurumi's nationalistic beliefs; if anything, he wishes to uphold them. Personally, I think it is very clear that Koito is not a 'good' person in the way the protagonists are, and that's ok. He's an antagonist for a reason, albeit a sympathetically depicted one. It would be unrealistic for every military-aligned character to suddenly give up the only life they know and hold hands at the end! (Of course, while reading, I always hoped he would leave and just join the circus or something in the epilogue...)
But it's undeniable that the ending we received not only makes more sense, but is appealing, especially for KoiTsuki fans. For Koito to be scarred by war and step up to take responsibility as a leader, commanding Tsukishima to stay by his side, and the mind-broken and exhuasted Tsukishima who no longer wishes to think for himself to simply fall in line and accept this, as if they're still bound by Tsurumi's presence, is moving...