I've created a system which allows you to generate NPC personalities during play - that is, while your PC gets to know an NPC, that NPC's personality is randomly generated, piece by piece. This way, you don't know in advance what such a NPC will be like.

And if you also use the optional Love system, that NPC may even fall in love with your PC - if you happen to be compatible with each other, which is never a guarantee. There is also simple mechanics to support courting in various forms (yes, even a little bit of that - although, the details are up to your imagination).