There are four strikers in the game.
Knives: The default striker for all characters, except herself
Mr. and Mrs. Chau: Are standard strikers for Knives. Which one appears is random, so just summon strikers with Knives, until you had both appear
Nega-Knives: She is the fourth and last striker that can be summond. Problem is, she can only be summoned by Nega-Scott. Which is a hidden character, that needs to be unlocked himself first. To do so, you will need to beat the game once with each Scott, Kim, Stills and Ramona.
Once you have unlocked them all and called each of them at least once, the Achievement will be earned.