If you’re confused why she is so dense, over 20 chapters (not an exaggeration) were dedicated to the Black Haired Girl realizing her love for Mito-kun.
We saw so much progress from BHG that she was about to confess during the Sports festival Arc. We’ve seen the whole emotional journey, from the spark to the realization and almost the confession. But the Main Girl is still at “I want to talk to you more Lieutenant Dan!”.
It really makes you wonder what was going on with the author, I suspect this was a forced editorial decision to add a second girl and the author went “Oh yeah? I’ll make her the main love interest to spite you!”