The Whales are the Five Voices of U, and the watch over Belle.
- While we only see four whales in U, the fact Belle rides them every time she gains untold popularity or attention does speak volumes that they are more important than mere story props.
Kei and Tomo's father has Hemophobia.
Kei and Tomo's father doesn't appear to care about beating his kids physically, but also doesn't seem to hold back when confronted with Suzu trying to protect them. It isn't until he accidentally causes a fairly large cut on her cheek and she turns around to face him that he sees the blood and backs off. It's entirely possible he has a fear of blood and backed down because of that and not the act of Suzu standing up to him. Kei and Tomo's mother protected them from their father when she was alive.
Their father was aggressive (or at least showed the potential to be) even before his wife passed, but she was the one who talked him down and/or protected them. Kei was forced into that role after her death. Seeing Suzu standing up to him was like seeing a ghost of his late wife, causing him to become fearful and back down.- It can also be likely that in some way, Suzu resembles her and it’s another the reason why their father doesn’t hit her.
Belle is set in the same world as Summer Wars
It's a decade or so down the road, and people have moved on from the previous super-hot app OZ to U, which is (hopefully) less vulnerable to hacking. See the flying whales. Kei and Tomo's father is Justin.
By the end of the film we never learn Justin's real identity, but he does share more than a few similarities to Kei and Tomo's father. Both characters become angry when other people don't follow the rules; Justin hunts down Dragon in U while Kei's father becomes violent with his sons for not listening to him. Justin also has lions as his animal motif in U, while in the real world Kei's father roars at Suzu when she is protecting his sons from him almost as if he is trying to act like a lion. This could tie into U's avatar system, as a person's avatar will reflect a person's desires or behaviors. The two characters even have similar body types when comparing the U avatar and the actual person. Plus much like how Justin's device to real a person's true identity is a silver lion bracelet around his wrist, their father similarly wears a silver watch.This could also explain why their father gets so upset when Tomo is singing Belle's song(s). By the end of the movie Justin sees Bell as an accomplice to the Beast for refusing to show him where the castle is. We also know that Tomo was Bell's first follower so he has probably been singing her songs since her rise to fame. If Justin and the father are the same person, his fits of rage could be triggered from hearing Bell sing and he abuses his children to stop them from singing. This could be seen when the Dragon gains new bruises after Bell sings "Lend me Your Voice" to him at the castle, as well as Tomo singing the song later during the live video call and Kei having to protect him; their father would have also heard it too if he was in the house. This could explain why their father backs down after getting a good look at Suzu when she turns around to face him, because as Justin he revealed Bell's true identity in U and now sees her again in the real world once again defending Kei/Tomo from his actions. He may just not have known that his son was the Dragon he had been hunting this whole time.
The father being Justin could explain why Kei/The Dragon was able to make and hide such a massive castle in U.
Justin was sponsored by the higher-ups in charge of U; it's possible his role gave him access to special areas and parts of U normal users couldn't reach. Kei somehow accessed part of that data on his own computer so he could make a place where he and his brother could be safe from their father, and they befriended the AI on the way to help them keep it a secret from anyone who would try to attack the Dragon. Peggy Sue is the girl that Suzu's mother died saving
Peggy Sue is the blue-hair U celebrity who gets annoyed at Belle's popularity but changes her mind at the Unveiling. She remarks "She's just an ordinary girl, like me". The girl that Suzu's mom saved was roughly Suzu's age. Maybe that girl is in high school now and she hasn't had any contact with Suzu's family (not wanting to remind them of the time that Suzu's mom died). Maybe she and Suzu will compare notes at some point and recognize their deeply personal connection. The reason Tomo liked Belle's singing and was her first supporter was because of the first song's lyrics
Tomo (aka the Angel) was the first to openly encourage and support Belle because not only did she have a beautiful voice and appearance, but the lyrics of that first song she sang, "Gales of Song", resonated with him. Suzu probably sang that song as a way of venting her feelings of grief about losing her mother. In the English version she sings "A life without you/I cannot accept/I can't tell that lie/I can't let go". Tomo, whom also (presumably recently) lost his mother could relate to her song. Following up the WMG that Tomo and Kei's mother protected them from their father and Suzu reminded him of her when she protected them, it's possible even before she was Unveiled Tomo noticed a resemblance in appearance/personality and was drawn to that as well. It might also explain why Kei told her not to look at him, perhaps she reminded him of his mother and it felt like his mother was looking at him, and Kei was ashamed of his behavior in U and/or the bruises on his back.