DINASTY WARRIOR 6

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Unlockable: Second Costume
An officer unlocks his/her second costume upon reaching level 25.
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Unlockable: Movie Player
Clearing Musou mode with one of the 17 officers who are available for that mode will unlock the CG movie player.
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Unlockable: Difficulties
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.
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Unlockable: Voice Gallery
An officer who reaches level 50 (the maximum level) will unlock his/her voice gallery.
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Unlockable: Characters
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

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Hint: Unlocking New Officers
Finish the mission targets to unlock new characters. This can be done in musou mode or free mode.
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Hint: Extra Height
Some officers can get a little bit more height from their jumps. Use a Strong attack while in the air to get a little more height. With practice, you can get most officers past some high fences the horse can't clear.

Since you can always summon a horse from anywhere (it teleports), you can use this to get across certain maps.
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Hint: More Experience From Officers
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.