Why do you think Aya calls her her best friend? And even bothers to think of her when encountering Yuu? Hikari is clearly important to her, even if in that moment her feelings for Yuu are more important.
Hikari's overwhelming shiny brightness is probably shown here as the start of how the story gets back to Hikari's part, where Aya is going to school, having friends, and is just the cool mature one of their friend group. Which going by the chats and the like, sounds like Hikari is treated like the cheery innocent one of the group. ETA2: The reason I'm emphasizing this is b/c Aya goes back to school b/c she's fed up after spending a month with her mom and is ready to bounce immediately and she ghosted Yuu instead of telling him about her home life. These glimpses of postive behavior only start after Hikari is in her life.
Eta: Saviour was probably an exaggeration, sure, but like Maruto named her "light" and I think that's the role she's supposed to be filling.
She's earnest. She clearly feels out of her element wooing Yuu but she still tries - hanging out with him and inviting him on dates and getting him to the festival. I'm curious w/ the current arc if it's the same story w/ Aya - despite getting shut down, she keeps coming back.
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