In the Scott Pilgrim vs The World: The Game, Knives Chau is your call in striker. Depending what system you play the game on, the striker is brought into play with the LB (Xbox 360) or with L1 (PS3).
For Scott, she flies in and exhales "LOVE" which knocks down and slightly damages all enemies.
For Stills, Knives throws down a row of numerous daggers that hit out in a long line to cause damage to enemies.
Kim's Knives strike is when Knives flies down to give Kim a quick kiss, restoring Heart Points at the expense Guts Points.
Ramona's striker is Knives appearing with a gigantic cup of coffee, causing all players to go into hyper mode and damage enemies.
Also available for striker play in the Scott Pilgrim game, are Knives' parents, Mr. and Mrs. Chau. They are unlocked by finding and defeating the random encounter of Mr. Chau on the Toronto game map. Much like the random Hammer Bros. appearances from the Super Mario games. When using the striker after Mr. Chau has been defeated, you also have a slight chance of summoning Mrs. Chau as well (though she is rare).
Knives is also a playable character, when you purchase the Knives Chau DLC for the games ($2 or 160msp) which also includes the 2 bonus modes, Battle Royale, and Dodgeball.