Protagonist don't need to change in every story. Protagonist is merely the person that moves the story along. I think you probably mean the main character. But even in that case, the main character doesn't need to change either. I believe that they do, however, need to change the world or change another pivotal character in the story. Like in your example, Belle's impact in the story was to change Beast. Her interactions in the story lead to a pivot in that world resulting in a new positive balance.
I wouldn't say it's a hard and heavy requirement that a protagonist, or main character, or whatever you want to call it changes--it depends on the story you're trying to write. If you're writing a story where your character has to grow and overcome some inner turmoil or self doubt to become the Hero everyone needs, they might need to change. If you're writing a story about some guy being awesome--James Bond, Jason Borne, Sherlock Holmes, Batman--they might not do much changing. It's almost entirely in relation to the story you're trying to write. The arc where a character changes is a tool to be applied to the right scenario, not a rule that must under all circumstances be followed. That said, my favorite stories are where characters adapt and change to overcome challenges. That's what moving through life is like.
A character having an epiphany is different from a character changing something fundamental about himself/herself. Andy Dufresne from Stephen King's Rita Hayworth and Shawshank Redemption comes to mind. Andy has an epiphany at the end of the story that he was not a good husband. He does not change. Ellis Red Reding. on the other hand, changes, courtesy Andy Dufresne. Even that change is not a major part of the narrative. It might be a challenge for a few people to write a flat character and keep him interesting. First thing would be to not give him a pov voice to the character. Keep him mysterious. You don't know what he would do next. His actions would be meaningful in themselves. That is the case where character change becomes unnecessary. Andy dufresne accomplishes a series of things being a prison inmate. So do John Wayne or James Bond. I think writing a flat character can be a good exercise for people who depend too much on character change in characters.
No. While a protagonist changing may enhance the story, it is not a requirement by any stretch. It isn't even necessary for the story to be GOOD. Sometimes it's a point being made that the character doesn't learn anything. Like @jannert said, Gone with the Wind is the best possible example of this. But, also, sometimes a protagonist is an isolated third-party to the story and is brought along as a "solver" -- this is especially popular in Golden Age-styled mystery fiction where the detective doesn't really have any stake in the book's narrative and rarely is the focus of characterization. Also, it sounds to me like your brother has a very narrow and kind of misguided idea of what character development is. A static character can develop, because development isn't inherently change or growth but just the rounding out of a character. As information and revelations about the character come to light, and their personality is expanded upon in detail, that is development, whether the implications of those revelations are that the character personally changes or grows or what. Sometimes establishing finally that a protagonist refuses to change is in and of itself a developing factor.
This is the keynotes taken from Writing Excuses podcast 12.45 (https://writingexcuses.com/2017/11/05/12-45-structuring-a-series/): Key Points: There are various kinds of series: one long continuous story, broken into parts (aka serialized); episodic, or continuing adventures of…; Ensemble series, with a cast and each book tells one person’s story; etc. Can you read it out of order or is there a required reading sequence? Consider the types of protagonists, varying from the hero who grows to the iconic hero who never changes.
The most recent example i've encountered involving a character that doesnt change is Koride in My Sister The Serial Killer Spoiler Koride begins as the average older sister who is hard working, plain, undervalued, and over looked. Her younger sister is gorgeous; their mother has such high hopes in her and spoils her. all men adore her. Koride always cleans up her sisters messes... including everytime her sister murders her lovers. Koride is always there cleaning up the blood and gore and disposing the body because in her day job, she is a nurse. The man Koride likes, ends up dating her sister, and this is where we START to see a change: Koride knows that every man her sister dates ends up dead. Koride doesnt want this to happen to this guy, but no one believes her when she says her sister kills people. they think she is jealous of her sister. Her friends and family turn on her, and the man she likes ignores her... until her sister attempts to kill him and he ends up stabbing her in self defense. The sister lives, and lies and says her boyfriend was abusive and attacked her. everyone believes the pretty sister and Koride, knowing the truth, SAYS NOTHING. ALSO, Koride has been confiding all of this to a patient who has be in a coma for years. She's talked to him every day since he's arrived. When he wakes up, he recognizes her voice and invites her out to dinner once he has recovered enough. Just when you think Koride MIGHT change..... she blows off the guy, and continues to clean up after her sisters messes as she continues to kill her boyfriends. NO ONE CHANGES. KORIDE IS STILL SLEF-DEPRICATING, THE SISTER STILL KILLS, EVERYONE STILL THINKS KORIDE IS JEALOUS OF THE SISTER. LIFE GOES ON it was SO frustrating to read, and the people i've talked to about this book who have read it say that it annoyed them and they hated it because Koride doesnt change in the end. So, PERSONALLY, i feel that some type of growth or change is needed. You can have a good read like My Sister The Serial Killer, where it builds and builds and you think something might happen.... and then it doesnt. it's just unsatisfying.
If the main character doesn't develop, then the story might end up being boring. But it's not always the case. You could have a character never changing their ideals, despite everything that happened to them, and that also makes for an interesting story.
