With most girls, giving them small prey is the best option. Others do have ways like talking to them at certain times or while inside them, one or two even accept money to give a huge boost in relationship. Some you currently can not build up any relationship at all.
Wendy: Starts at 20. Can gift micros for 1, small prey for 5. Can't build relationship while inside her.
Roxanne: Can't build any relationship, yet. Meet her at the gym in the morning.
Amy: Can build relationship while talking to her at the pool. Can only gift her micros or small prey if you visit her home during the weekends on the pred island. Also can build relationship up while talking to her while inside her.
Marlie: Can gift her money while she is at her shop, 1 point for $5, 2 points for $25, or 30 points for $500.
Amanda: Gift her micros or small prey at first, once up to 20 points, you can ask to be her prey for money when you can talk to her at various times to further build up relationship.
Soraya: Talk to her at the library or gift her micros or small prey. Can also build up relationship while inside her by talking.
Vezess: Visit her at the beach. Currently not a pred for the player and can't build up relationship
Seri: Talk to her while she watches TV in the morning and daytime, or gift her micros or small prey when visiting her room.
Serin: Gift her micros or small prey. Can also ask for a special room where she will eat you, can talk to her to build up relationship while inside her.
Cerlin: Can only visit in principal office, not a pred for the player, yet, can't build up relationship.
Diamante: Can gift her micros or small prey at noon in her classroom. Also can build up relationship talking to her after offering to be her prey.