Hyuga first appears to be very violent, a trait that seems to have been with him since middle school. He looks down on people weaker than him as weeds, and has no scruples about attacking people from behind. He describes himself as both serious and solitary, while Hansode Shiranui describes him as having a bad personality. After being "converted" by Medaka Kurokami however, Hyuga has mellowed out considerably, being much more willing to work with others. It is implied he has developed feelings for Medaka as well.
(Source: Medaka Box Wiki)