This is based on opinion. Please don't list it on a work's trope example list.
Tear Jerker / Gakuen Babysitters
When Kotarou is sick in the first chapter, Ryuuichi tries to call their father. That's when he remembers his parents aren't alive anymore and he finally starts to mourn over their deaths.
When Ryuuichi is sick in Chapter 7, he has a nightmare about the day that his parents died, only they also decide to take Kotarou along with them so Ryuuichi won't have to babysit again. Ryuuichi tries to stop them, but an invisible wall separates them and he's powerless to stop them.
Chapter 39, just all of it as it shows that Kotarou is afraid of losing Ryuuichi as well.