Successfully completing the indicated mission to unlock the corresponding girlfriend, which you can then kiss for a health bonus:

Beatrice: "That Bitch" mission in Chapter 1.
Lola: "The Tenements" mission in Chapter 3.
Mandy: "Discretion Assured" mission in Chapter 4.
Pinky: "Carnival Date" mission in Chapter 2.
Zoe: "Smash It Up" mission in Chapter 5.