Parallel reading on manga and light novel -- which means light novel #2 and manga #2,3, and 4. Spoilers ahead for the earlier volume -- and each one may spoil the manga before.
The light novel, and the first manga, open with the arrival at school that was the beginning of the dating sim game (with the novel having more backstory about her preparations for this day). And at once, Katarina shows the same failure to notice things as in the first volume: when Jeord, her fiance, meet the game's heroine, Maria, climbing a tree, she remembers only that was the start of their romance in the game, and caught up in that, does not hear how he and Keith, her adoptive brother, jest about how much forthrightly she acts when she's caught climbing a tree.
Katarina, over the classes and the summer break, does miss other obvious stuff -- poor Keith comes across as censorious for trying to urge some manners as their mother would approve of (plus, of course, jealous) -- though she does have the sense to befriend Maria to try to ward off her Catastrophic Bad Endings. (Never realizing, for instance, that the endings were caused by her reaction to Maria's romances, not them in themselves -- again, sometimes it's comic, but not often enough to my taste.)
The third manga opens (and the light novel continues) with a scene where Katarina is, much to her shock, accused of bullying Maria. Just as in the game, except that the accusers are now some girls jealous that she's engaged to Jeord, not the main characters of the game -- and the main characters of the game, all being her friends, come to her aid.
And then Maria vanishes, and Jeord discovers that the inexplicable accusation had much darker possibilities. Which pan out.
Leading to the opening of the fourth manga (and the continuation of the light novel) with Katarina in an inexplicable-to-the-characters sleep of days, and a touching scene where all of her circle, boys and girls reflect on how much she means (one realizing what he never had before). There's a plot device that was a bit foreshadowed but still felt clumsy, but she wakes ready to deal with discoveries with her usual style. And then boldly faces the party where the games ends, with plans for any number of Catastrophic Bad Endings.