I don't know what i've done wrong but it looks like Hana has a thing for Bloodfang more than my character lol even tho i've been treating her nicely. Not sure i've done anything that offend her unless giving tips to the dancer and walked into the brothel asking about girl without doing anything pissed her off lol. Are the npcs able to romance each other in this game? Sorry for asking too many questions but i find your game intriguing despise some bugs.
If characters get annoyed with something you do, they will comment on it, so don't worry about secret parameters. As for Hana, at the start she is just nice to Bloodfang, same way he is with you. The NPCs can have relationships with each other, though that depends on your actions. Particularly, if you do missions with the other girls, or if you romance Hachi enough, Hana will be in a relationship with Bloodfang.
Keep in mind Hana (specifically her) is not monogamic. Even if you romance her to the max, she will still occasionally flirt with other characters (more often than not to your advantage). You will understand her character better as you go through the game.
Well, i havent done any other girls mission or even met Hachi yet. After giving money to the blind elf in an Asari's bar, a cutscene playing and Hana and Bloodfang already blushing at eachother lol, is that normal? Should i still avoid other girls mission in the future or its already too late xD. Tbh i found her and that "mother" and those 3 demons they called trinity all sketchy. Still i wanted to romance her for the first playthrough xD. Also how do i get into the room on the left?
Edit: i slept at an inn on floor 6 and a cutscene played as mother goat tell Hana to seduces Gouki :| and she reluctantly agreed. Should i even keep romancing her at this point since i smell some love triangle drama lol.
Edit 2: Does the quest for the succubus killer supposed to be done on floor 6 or later? Also on floor 6 there're a loop map called the Nobleman's farm that have 3 demon roaming around. I killed all 3 of them but after that i found absolutely nothing more to do there. The door lead to the mansion is locked and it said no one is home.
Edit 3: Now that i know that Hachi is the redhead, then who's this girl 
who sometimes pop up in conversation as if she was a party member even though she's not?
Sorry for taking so long to respond. I wanted to focus on the new version of the game's release with the Nao mission, and also Dota released the Battle Pass :P .
For main post: That mansion used to be a whole self-restrained horror mini-game, where Hana (with her old faceset) explored a haunted mansion alone. Since then though, both the plot and Hana's personality have been rewritten from a scratch retconned so much and my techniques and tools for creating the game been upgraded so much, that it would take more work to fix the mansion than to recreate it completely. Right now I'm not sure what to do with it. One idea is to create a mission exclusive for Hana if the player has romanced her enough, which will also be a self-contained horror game, but with the modern plot and her personality in mind. Another is to keep it in standby, for one of the class girl missions. A third idea is to scrap the entire area entirely, and create a new area with a new event/boss, like the rat village or the zombie village. I'm open to suggestions.
For Edit 1: Well, Hana is not exactly "bound" by monogamic limitations. In her head she is the hero of the story, not you. She and Gouki will just remain friends (unless you follow one of the other girls).
For Edit 2: The quest can be finished as soon as you reach Floor 6. Depending on how you choose to finish it, more quests might unlock for later floors, but the first part is entirely done on floor 6.
For Edit 3: The girl you are seeing is Hana's old face, before it was replaced. I have changed almost all of them to her current faceset, so do tell me when you find one of them.
