Here's a quick (not complete) guide I've made on Befriending succubi. Each entry starts with the conditions needed to Befriend a succubus, followed by useful Conversation tips:
HOP Member: Investigate the HOP main church (currently impossible).
Succubus: Visit Mirror Town.
Lesser Imp: Use Pillow Talk > Provoke, ejaculate at least once, then use PT > Beg for mercy.
Aplysia: Reto must wear a Slimy Bracelet.
Bikini Succubus: Travel at least 2000 m. Talk > Praise her breasts is good but puts you into a Heavy Hold.
Sea Succubus: Use Observe. Talk > Praise her breasts is good but puts you into a Heavy Hold.
Deep Sea Succubus: Mirror Presence must be at least 50. Pillow Talk > Embrace + Motorboat her breasts but puts you into a Heavy Hold.
Invy of Envy: Visit Mirror Town with Prosperity Level at least 30 (impossible).
Training Succubus: Reto's Stamina must be 50 less than it was at start of battle. Action > Hold hands + Joke > One-shot gag are good, + she also likes Present > Super Spicy Chicken.
High Succubus: Reto must have the Spades attribute. Present > Colorful Bouquet maxes out Love Voltage instantly.
Cow Girl-chan: Talk > Chat > 1st option + Talk > Praise her looks are good but give Reto Lolicon.
Cow Girl: Have any milk item. Talk > Praise her breasts is good but raises Lust.
Nekomata: Reto must have Fishy status at start of battle.
Magic Imp: Reto must have Lolicon status at start of battle. Action > Approach her + Action > Body touch are good but raise Lust.
Lethargic Succubus: Ejaculate at least once, then let her be sleepy for 1 turn. Pillow Talk > Pamper + Pillow Talk > Motorboat her breasts are good but put you in a Hold, + Colorful Bouquet is good.
Necromancer: 10% chance of being befriendable if you have a Hard Bone, 40% if you have a Strange Bone. Action > Stare is good.
Est of Sloth: Visit Mirror Town with Prosperity Level at least 30 (impossible). Stuffed Cat + Adult Toy are good presents.
Symmetry: Lewis or Carroll must have the Beloved state and the same level as Reto.
EDIT: Some tips on where to get presents: Lesser Imp sells Colorful Bouquets and Magic Imp sells Stuffed Cats, once they're befriended. Adult Toys can be dropped by Necromancers. These presents are a bit expensive, but it's worth having a few of each in your inventory.