Fruits Basket S01 E10 Make It Clear If It's Black or White

Tohru's class goes on an endurance run. In the middle of the run, a white-haired boy, not much younger than Yuki, interrupts and challenges Kyo to a fight. The boy is also a member of the Sohma clan. As they fight, Yuki feels faint. What will Tohru do now? And which zodiac member is this boy?