All players will start as Academy Students, exceptions are devs. Players are given sprite bases to customize their own characters
As you are customizing you character, note that you have limitations, no starter players can have Headbands or symbols on their characters, Jinchuriki are chosen by me and me alone (Tsumi), Everyone needs to take exams in order to rank up or be taken into a bloodline.
The only time you can customize an adult character is when you reach the rank of Jounin, Academy Students are prohibited from having Senseis, as this gives them too much of an advantage in future battles. Sensei's can be appointed to you by a Kage or you can ask one to train you.
When you become a Jounin or a Jounin Sensei you can now develop your own Jutsu that can only be used by either you or a person you taught it to. As a Jounin Sensei you can also choose to become an Academy Teacher.
Despite these limitations you are given full control over your character, their personality, clothes, and how they get stronger is all up to you.