His very strong push swept Sei off her feet. And she was supposed to still be in love with the story's protag but his bold gambit managed to edge that guy out of her heart. That was fast. She was supposed to feel some hurt talking about her original crush with her friend who her crush loves, but apparently that pain has dulled for her knowing there's someone out there who loves her and is willing to break her fall. Admittedly that's not so realistic, but hey this is fiction.
Tsukasa may have been an otaku originally, but he's also a natural at rizzing girls. He's gonna give his new little sister a brother complex at this rate, don't rizz her too! Knowing his new sis is also originally a losing heroine instinctively made him wish for her happiness. Best part for me is when he decided on pg. 22 not to turn back on his confident confession full of conviction, even after realizing this is now real life and not some dream.