Hi! My Project in here should do the explaining for you! :)
But.. Just in case...
So you can create a whole new character or give more info/ write a backstory to an existing one e.g Horace Slughorn, Helga Hufflepuff etc. I personally think it would be really cool to do a little bit more about the founders of Hogwarts, but there you are! Hope that helped.
Ps: If you don't know any Harry Potter characters you can ask me if you like or look some up on the internet.