OH! O _ O I thought that most likely Aika got that part as she was the youngest/"newest" member before 9th gen but now she isn't so it almost brought attention to her...
I mean- before the JunLinEri graduation, Aika wasn't really a centre girl or having leading lines- so it almost brings her forward slightly as one of the older 5 girls- since she's a couple of gens below the 5th and 6th gen remaining and quite young. At the same time though- that also makes her stand out a fair bit (possibly too much? D: ).
I've also been thinking that a fair bit when they show her helping/lecturing 9th gen in rehersals, like they're making an effort to show her teaching them- but that could also just be Aika herself making more of an effort to help (it's her first time with younger girls/gen so she's taking advantage of being able to help/boss around the girls
)