やがて君になる [Bloom Into You] Series by Nakatani Nio

Student council president Nanami Touko is in love with her friend, Koito Yuu! Even though Yuu doesn't love her back, she stuck by her side and joined the student council, too. She's accepted Touko's feelings -- but what about her own? Yuu still doesn't have anyone she likes... but now, a new thought is growing her her heart: "I want to fall in love with Nanami-senpai!"

They might be on vacation, but the play is still on everyone's minds. Touko is already starting to worry about what will happen after it's over, and Yuu wants to change the script's ending. Stressing about the play isn't the only thing they're doing, though. It's summer break, after all -- so Yuu invites Touko out to the aquarium!