I was hoping this part got rewritten in this version. It really doesn't paint her in the best light.
The important part to remember, at this point in the story, is that you are only seeing specific flashbacks. In the conversations, that's where the bulk of the backstory comes out.
MC looks to have been with Keyla seriously, prior to Kim and Alessa. She left with her family to Japan and then they moved back to Buffalo. After Keyla left, MC went with Alessa; the MC's mom even asked about the correlation between Keyla leaving and him seeing Alessa.
While Keyla was in Buffalo she met Zeke, and they dated. When they returned to New York for Zeke's schooling, she realized Zeke was MC's best friend. MC and Keyla were sneaking around on Zeke around this time... Was it right? Nah, but it was just as much the MC's fault as Keyla's.
At some point the MC and Kim started getting close. Keyla decided to break up with Zeke, but he surprise proposed to her (for an as of yet undisclosed 'desperation' reason). Seeing as the MC was no longer available, she panicked (I guess?) and accepted the proposal instead. This decision is entirely on Keyla.
Even Zeke admits that it wasn't a great decision, due to his desperation. I'm curious to see what this was to force him to propose to someone he didn't seem to be fully into.
Was it all handled perfectly? Nah. But the blame on some of the decisions is on MC's just as much as Keyla.
When it comes time for Kim to leave, Kim asked Keyla not to tell the MC. Keyla obvious agrees to this, which she later admits was also related to her selfish reasons. So it was probably a bit of abiding by the promise she made as looking forward to Kim actually leaving.