All Characters
Have a saved game file from Samurai Warriors on your memory card to start with all characters unlocked.

In-game Reset
Hold Start + Select during game play.

Alternate Musou Attack
When your Musou meter is full, press Circle to begin the attack. Then. release Circle and press Square rapidly. Your character will stop their regular Musou attack and hit the enemies with stronger versions of their charge attacks. This is especially useful with characters such as Nobunaga, that have erratic Musou attacks that only work against large groups.

Green Cursors
A green cursor around your character at the character select screen indicates that specific character is a 'lucky' character. These characters can receive there fifth and sixth level weapons one difficulty below the supposed setting.
Every time the game is turned off and restarted, the cursor will randomly change to another character.