Danganronpa Headcanons
Theory: Part of the reason Hiyoko bullied Mikan the most was because she was envious of how Mikan was able to teach herself her skill. Evidence: It's stated that Mikan taught herself how to heal her own wounds after being abused.
What did Hiyoko do to Mikan?
However, deep down, Hiyoko actually does care for Mikan. For example, in Danganronpa 3 manga, it was revealed that Hiyoko accidentally hurt Mikan when she and Mahiru ran into her while walking to school.
How does Mikan feel about Hiyoko?
Despite Hiyoko's bickering and attitude, Mikan still cares for Hiyoko and states about her health which however Hiyoko refuses to listen to Mikan, like in Chapter 1 when Mikan tries to tell Hiyoko that eating sweets for breakfast is bad for health.