Besides, I think this provides a good segue (good as in storytelling and not friendship) to the theme of Yumi leaving the nest; she finally lands a job, finally makes a new friend, and likely lives all by herself now. It's an appropriate time to have Yumi depart from Chika to some extent despite the fact that it's not done on a positive note.
This is a very good point. And also there's an impression, although it might just be me, that Yumi (without even trying) is and always have been closer to Akira's heart than Chika. She certainly is where the universe is concerned, and I wonder if Chika will put two and two together on that front as well.
Because Chika's relationship with Akira has kind of always been about her doing everything she can to please him (it's just that she follows what she thinks will please her "image of a boyfriend", instead of actually looking at Akira), and I think that if she now stops to think about how this girl, that literally does nothing to cultivate that affection, has it (certainly right now after the coma) more than her... well, that by itself might spawn some jealousy.
It's like: "I love this person and I can be with him to a degree, but now I've found out that the only reason I can is because my sister (Yumi) is insistent on not acting their mutual feelings, otherwise I'd probably lose to her. I mean, I'm already losing and she's not even trying for my sake".
I feel like a scenario where she realises that Yumi has
not been acting against her, but keeping herself back for her sake and still winning could be even more damning for her self-esteem.
