It's been a day since the final episode, so I'll include some spoilers.

There have been hints throughout the story about whether Aqua is the one, or whether Aqua or Akane will be the one chosen, but...
Aqua is actually Dr. Goro inside.

He's Sarina's doctor. He declared that he would become the one to help Sarina achieve her dreams.

Ruby lost her beloved Ai after a family conflict. She was depressed from the start of season 3 after losing Goro. But with the confession in the final episode, it feels like she was freed from one of her burdens.

Final Episode Supplement (Contains Spoilers) | UTA's post