first off about the game; I found that the narrative sometimes juggled indefinitely between tenses, which made it little confusing to follow (this particularly applies to other works that I played earlier too). There were also typos etc. Some plot points (like in one about that asking if the protag was familiar with that detective movie) and for a standalone if I'm not wrong felt a little abrupt, unexplained due to the fast pacing, it lacked like... solid ground basis for how could he and why he acted so in those 'off' moments aside from being his traits counted as part of mystery lore or something, the cat piano scenes and ends were hilarious though. Additionally, while the art was appealing overall, some CGs felt a bit stiff compared to the rest, butt some were charming enough in its simplicity. And maybe it could be enriched more by SFXs, something along them, aside from the soundtrack, overall just need polishing this one can come out greater or just perfect.
I can see how much you've refined these aspects in your newer works, and it’s great to see that impressive progress! If you’re ever revisiting this or working on something similar, a bit of extra proofreading and smoothing out pacing could make the experience even stronger. Which, I'd be happy to help out some on that! I really enjoy your works, and I'd love to support however I can.
lastly, this was just my theory and had been in my mind so I wanted to be sure, but is Tantei inspired off of.. you know those protags from DV3? He was talking almost as if he was the child of them. Considering the name, his appearance, the sprite design here reminiscent of the characteristic style, and added with that fangame you made, it could be like an OC you reserved for that universe, related with those other characters and references, Easter eggs that made way into your current one too, how was the chronology and stuff. So far I noticed there recurrences with Ramu, Mitsuki, his sister, Noreki, maybe more that I missed...