Successfully complete the indicated task in Story mode to unlock the
corresponding character:

Diao Chan: Successfully complete the "Battle Of Hu Lao Gate"
mission on the Five Star difficulty.
Dong Zhuo: Successfully complete the "Battle Of Luo Yang" and
"Battle of Mei Castle " missions on the Five Star difficulty.
Lu Bu: Successfully complete Chapter 6.
Meng Huo: Successfully complete the Nan Zhong requests in Chapter 3.
Yuan Shao: Successfully complete the "Zenith Tower" Request
mission in Chapter 6 on the Five Star difficulty.
Zhang Jiao: Successfully complete the "Yellow Turban Rebellion"
mission on the Five Star difficulty.