An officer unlocks his/her second costume upon reaching level 25.

Clearing Musou mode with one of the 17 officers who are available for that mode will unlock the CG movie player.

An officer who reaches level 50 (the maximum level) will unlock his/her voice gallery.

Clearing Musou mode once with any officer will unlock Expert. Clearing Musou mode with at least one officer from all factions (Wei, Wu, Shu, and Unaffiliated/Other) will unlock Chaos difficulty.

The following characters can be unlocked via the techniques listed.

• Cao Cao - Complete Musou mode with three Wei characters
• Cao Pi - Defeat Cao Pi in less than five minutes on the Wu side of He Fei
• Cao Ren - Complete Battle Objective 2 on the Lu Bu side of Fan Castle
• Diao Chan - Complete Musou mode with one character from each kingdom
• Dong Zhuo - Complete Musou mode as Lu Bu
• Gan Ning - Complete Musou mode as a Wu character
• Guan Ping - Complete Battle Objective 2 on the Shu side of Fan Castle and finish the battle with Guan Ping surviving
• Huang Gai - Complete Battle Objective 3 on the Wu side of The Battle of Chi Bi
• Huang Zhong - Complete Battle Objective 1 on the Wei side of Ding Jun Mountain
• Ling Tong - Complete Battle Objective 1 on the Wei side of the Battle of Shi Ting
• Liu Bei - Complete Musou mode with three Shu characters
• Lu Bu - Complete Musou mode with Liu Bei, Cao Cao, Dio Chan and Sun Jian
• Lu Meng - Complete Battle Objective 2 on the Wei side of He Fei
• Ma Chao - Complete Battle Objective 2 on the Wei side of Han Zhong Attack Defense Battle
• Pang Tong - Complete Battle Objective 3 at Cheng Du battle and finish the battle with Pang Tong surviving
• Sun Ce - Complete Battle Objective 1 on the Sun Jian side of the Battle of Xia Pi
• Sun Jian - Complete Musou mode with three Wu characters
• Sun Quan - Complete Battle Objective 1 on the Lu Bu side of The Battle of Chi Bi
• Taishi Ci - Defeat Taishi Ci in the Battle For Wu Territory
• Wei Yan - Complete Battle Objective 2 on the Shu side of Wu Zhang Plains and finish the battle with Wei Yan surviving
• Xiahou Yuan - Complete Battle Objective 1 on the Shu side of Ding Jun Mountain
• Xiao Quio - Complete Battle Objective 3 on the Wu side of the Battle of Shi Ting
• Xu Huang - Complete Battle Objective 2 on the Wei or Wu side of Fan Castle
• Xu Zhu - On He Fei on the Wu side, personally kill Cao Ren and Xu Chu
• Yue Ying - Complete Battle Objective 1 on the Wei side of the Battle of Wu Zhang Plains
• Zhang Jiao - Defeat Zhang Jiao in the Yellow Turban Rebellion
• Zhang Liao - Complete Musou mode as a Wei character
• Zhen Ji - Complete Battle Obective 1 on Xu Du Invasion Battle, Lu Bu scenario
• Zheng He - Complete Battle Objective 2 on the Shu side of Han Zhong Attack Defense Battle
• Zhou Tai - Complete Battle Objective 3 on the Shu side of Yi Ling
• Zhuge Liang - Complete Musou mode with a Shu character

Finish the mission targets to unlock new characters. This can be done in musou mode or free mode.

If you want more experience when killing various officers, use your grapple attacks. Doing this will net you more experience (sometimes twice as much) than you would normally get.

Addition by KnightMask:

You can also get more experience from killing officers if you're using a special attack as they die (as in, the attack that kills them). Additionally, playing on harder difficulties further increases experience points earned.