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

Effect - Code

Beatrice as a girlfriend - Complete the mission 'That Bitch' in Chapter I
Lola as a girlfriend - Complete the mission 'The Tenements' in Chapter III
Mandy as a girlfriend - Complete the mission 'Discretion Assured' in Chapter IV
Pinky as a girlfriend - Complete the mission 'Carnival Date' in Chapter II
Zoe as a girlfriend - Complete the mission 'Smash It Up' in Chapter V