Su Yu was originally a girl.
Later she became a vampire.
Later, she traveled through time and became an... Emm little boy?
——
Relax into fantasy writing.
As for the gender of the protagonist... It's actually not very important.
After all, he is the protagonist with a genius halo! There is the beautiful Suzaku brother who is standard in the contract stream and the shameful flower demon who attacks the master every night!