Tbh kitagawa falls for him super fast so pretty much every chapter has flirting. Gojo doesn't flirt back purposely since he's super awkward and shy and for a long time thinks they're just friends and he's got no shot. The latest chapter shows just how dense he is, so it takes a while. In terms of important chapters for their relationship, the mid to end of the coffin party group cosplay, and even bigger, pretty much the entire haniel arc. I'm not 100% sure but I think those happen around chapter 100 to the current. I honesty recommend you just read the whole thing. The chapters are each very different and interesting so for me time just flew past as i read (and later reread) the whole thing in like a few hours. You won't understand whats going on with those chapters without reading the earlier ones since they contain very important characters that they meet over time in the earlier chapters. Especially for the coffin party group cosplay. Seriously just read it, its really good and more than worth it.
Kotagawa flirts with him like, all the time xd. Is one of those situations that scalated real quick, you kind of have to read everything anyway.
Lmao then which chap did the anime left off
Chapter 39 is where the anime ended, so from chapter 40 onwards is after the anime. I just want back and checked so that should be right.
Someone needs to tell me the chapters where they flirt (just the important ones) cause i dont wanna read allat