Most obscure ones in my opinion are:
Captain- Visit the ship at night (can make it night either in Downtown or Green Town, and for some reason the Captain has a male sprite).
Scarecrow- After beating Zombie Village, talk to the scarecrow a few times.
Zombie Girl- After beating Zombie Village, she is found in one of the houses (even Evil is disgusted)
Goggles- When she is in your party, sleep at Capital's inn.
Prostitute- In Downtown at night, there is a a building on the bottom with hearts on the side. Costs 2500G.
Fair Prostitute- In Downtown Brothel at night. Need to have less than 2500G but more than 250G (can deposit excess gold in Merchant Guild). There are two parts to this scene.
Travelling Merchant- Happens to be the Merchant Guild Leader. Can have the scene with her after being introduced to the '???' cave a second time. She is in the Merchant Guild in Capital.
Queen- Insect Queen in Unexplored Lands; can have the scene with her after defeating Blue Sorcerer.
Mercenary Leader- Do her whole quest (defeating the giant lizard) and talk to her.
Heelin (Slime)- After doing her quest (can find the guide on page 3 of this thread).
Slave- In the basement during the Mercenary/Adventurer raid. Nobody is fucked up enough to do it hence why its obscure.