[A Gathering of Heroes]
[All officers are playable]
This will unlock when you clear the TRUE ending of the game, as you need all officers aside from the Orochi ones playable before the final level will unlock. Every character is unlocked simply from clearing stages (excepting the modified stages where you must interact with the character you saved, and ONE major exception) but certain Gaiden stages will require friendliness between certain characters to be higher than level 1 (aka get them maxed in red, then play 1 battle to get into yellow) before unlocking.
Chapter 2 ????? (Battle of Jian Ge) - Motonari Mori, Jia Xu
Chapter 2 ??????? (Battle of Mikatagahara) - Lian Shi, Lu Meng, Masanori Fukushima
Chapter 2 ????? (Battle of Tao River) - Goemon Ishikawa, Hideyoshi Toyotomi (Lian Shi must be unlocked)
Chapter 2 ?????? (Tetorigawa Machine Battle) - Zuo Ci, Benkei, Xu Huang
Chapter 2 ????? (Battle of Liang Zhou) - Liu Shan, Guan Suo
Chapter 2 ?????? (Battle of Xing Shi) - Ginchiyo Tachibana, Xiao Qiao, Sun Shang Xiang (Chapter 2 ????? [Battle of Kyuushuu] must be cleared)
Chapter 2 ????? (Nanzhong Extermination) - Zhu Rong, Meng Huo, Zhang He
Chapter 3 ????? (Battle of Jie Ting) - Xing Cai, Keiji Maeda
Chapter 3 ????? (Battle of Guang Zhong) - Sima Yi, Masamune Date
Chapter 4 ???????? (Battle of Komaki Nagakute) - Zhuge Liang, Huang Zhong, Hanzo Hattori
If 3 names are listed, each of those 3 must have level 2 friendliness with the other 2.
That one exception is Lu Bu. To unlock him, first unlock Diao Chan, then use her in your party and play Chapter 3 ?????? (Battle of Osaka Castle). When Lu Bu appears, you must run to him before he retreats from Ne Zha. You'll know you did it right if he says a new line to you and Diao Chan responds (and then he will turn from yellow to blue as an ally).