Well remember we have customization, it's possible that the hat is part of the original customization we can use. As for the hair style, they proably did it to make one be able to know which is the main character (as in the player character) and which isn't or something like that.But how they can be different from the hero you choose ?
I heard Xavier and Yvonne are the default name you get when you choose your hero, while Calem and Serena are the name for your opposite gender rival which is your neighbor. And I think later Shauna let you choose a nickname for your hero. So at the start you are either Yvonne or Xavier then Shauna ask you which nickname you want to be called
Edit:
https://bulbapedia.bulbagarden.net/wiki/Serena_%28game%29
So real name is Serena ? because never saw an official reveal telling the protagonist was Serena, even the official website don't show her as Serena for the protagonist part only the NPC one.
Calem and Serena are the official names of the X & Y protagonists, going by what Bulbapedia claims.
Well Bulbapedia updated, and says that Xavier and Yvonne were their pre-release media names just like Whitlea and Blair.And later Shauna ask you for a nickname and she call Xavier, X-Meister. Since they keep doing something new with this generation, I'm sure the protagonists will have different names than your neighbors NPC one. Also Calem and Serena on the official website ( Us and Jap ) are only named in the new friends section, while the Protagonist section still have Hero and Heroine.
Some screenshots show Xavier and Yvonne, if their name would be Serena and Calem I'm sure the screenshot would use these names instead, since they got released at the same time than those names.
According to their Bulbapedia pages, Xavier and Yvonne were just prototype/fanmade names, sorta like how Hilbert and Hilda were once referred to as Black/Blair and White/Whitlea until the final names were revealed, or how Lyra was called "Soul" until her name was revealed, and Ethan was referred to as "Gold" until he got renamed Ethan in HG/SS. Same also went with how Nate and Rosa were referred to as "Black2" and "White2", as well as even "Ebony" and "Ivory" for a few months. Going by that pattern, it's not uncommon for protagonists to get referred to by a name of the games, they usually call the boy by the name of the first of the paired games (Such as Ruby, Diamond, Black etc) while the girl was often referred as by the second paired version (Examples including Sapphire, Pearl, White) until the official names came out.
In fact, prior to people calling them Xavier and Yvonne for a bit, Calem and Serena were also once referred to by fans as "X-chan" and "Y-chan". Calem and Serena are the official names of the X & Y protagonists, going by what Bulbapedia claims.