Completing the following missions will unlock a certain girl to be your girlfriend. Note: enabling you to kiss them, thus receiving a health bonus.

Girlfriend	How to unlock
Beatrice as a girlfriend	Finish the mission 'That Bitch' in Chapter 1.
Lola as a girlfriend	Finish the mission 'The Tenements' in Chapter 3.
Mandy as a girlfriend	Finish the mission 'Discretion Assured' in Chapter 4.
Pinky as a girlfriend	Finish the mission 'Carnival Date' in Chapter 2.
Zoe as a girlfriend	Finish the mission 'Smash It Up' in Chapter 5.