So, I saw that a lot of people still have problems and don't know how to trigger the secret end. I am going to make a list of what bosses you need to defeat and how you need to defeat them. Basically, this is going to be a walkthrough until you trigger the secret end. After that you need to find stuff on your own
The first thing you do before anything else is to unlock the "bad boy" character. You do that by buying gasoline from the demon vendor. With the gasoline you can get rid of the motorbike at the bottom right corner of the map. Go down the rope and unlock the "bad boy" character. After that, make your way towards Ayumu (you can get there by using the bottom right path of the school map). Using the first MC will trigger the succubus phase for Ayumu. Make sure to defeat Elfriede before you fight Ayumu (use the explosive barrel to unlock Elfriede).
Once you defeated Ayumu and died to her, you can choose your next MC. Take the "bad boy" you unlocked. His special ability is to break doors and windows. Before doing anything else, you go back to the map where you go to the Ayumu boss fight. There is a brakeable door. Destroy it and unlock the "long-haired blond guy". Now, go to the demon vendor and buy the mallet. This allows you to break the door leading to the secret room where you can rechallenge all the bosses you defeated so far. We need that room later on. Next, you do the area with the green-haired girl. You get to that area by going through the mirror at the bottom right corner of the school map.
Do that area and defeat Succubus-chan, before fighting the green-haired girl. Succubus-chan herself is easy to unlock in this area. Just use the mirrors until you get to her. You might have to test around a bit until you find the correct mirror. So make sure to save at the begging of this map. In order to trigger the green-haired girl's succubus form you need to pick the dialogue options during her fight. You can't miss it.
The next MC you pick will be the one you unlocked before (long-haired blond guy). With him, go to the bottom left corner of the school map. Talk to the dude chilling there. This unlocks the next character you use. Then, go to the demon vendor's area. There is a glowing object on the ground. Go to it, pick the right text option and then go to the basement (if the object is not there, you have not unlocked the basement correctly. So do that). Now, you can talk to the angel girl. Go through her dialogue until you trigger the boss fight. You fight her, you die and on to the next fodder character.
Pick the swimmer guy you unlocked before. Now, go to the demon trader and buy the axe. Then, go to the bottom right area and enter the first level of the next boss map. At the top right of that level there is a door you can enter once you have the axe. Do that and you can talk to another MC, unlocking him for later. Now, just do the areas until you get to the area before the Mermaid Girl. There you can go to the pool on the middle of the map. "Talking" to it will unlock a door. This leads to Sophia, your third notorious character. After defeating her, go and do the main boss of this area.
Now, you have 3 notorious characters down, 3 main bosses and the angel girl. Next we go for Est. You can unlock Est by picking the guy you unlocked before (he is wearing a beige uniform). In order to trigger Est's (the nurse) fight, you need to talk to her while having a full pleasure bar. So fill that bar up and then talk to Est. Pick the "...." option and you will trigger the first "fight". After this, Est will talk to you again and you can unlock the actual fight. You win, you die. You lose, you die. Either way, you are a happy man cause Est is awesome.
Now, we go for the rabbit girl succubus. The character you need to choose for her is the orange haired guy. Pick him and make your way towards the top right corner of the school map. Enter the new area and follow the path and maps until you get to a door on your right. You can't get there by doing the normal jumps. But you can use your "Q" button to active your super jump. With that, you can jump over the two blocks, leading to a secret area where you can find your last MC. After that, go back and continue with the map. Eventually, you will get to a map with a yellow button. By jumping on that button, you unlock the path that leads to Alma Elma. Defeat her and after that, make your way towards the area's boss (bunny girl succubus). Her succubus form triggers by itself. So do not worry about that.
Now, we only have three enemies left. First we will take care of the prostitute. In order to do that, the next MC you choose will be the nerdy guy you just unlocked. Then, go to the prostitute (right side of the school map). You need 10k in order to unlock the first "fight" and after that you unlock the second fight. The second fight, just like with Est, only unlocks if you have the correct character. So if it does not, you fucked up somehow.
Finally, we can choose our last character. You already unlocked him at the very beginning when you talked to him in order to summon the demon vendor. There is not a whole lot to do at this point, except for two things: The last two succubi and the secret scene in the basement. First, go for the secret scene. For that, you need the lost grimoire. You get that by unlocking one full path on the ability map. Once you unlocked the last ability, there is an item you can pick up. Take it and go to the basement. Talk to the candle in the middle of the room and watch the secet scenes. Now, just make your way to the final area and proceed until you get to the room with all the burning candles. The candle at the bottom right corner is not lit. You can active that one by using one of the fire dudes in the room. Just let him die next to it and there you go. This unlocks Fulbeua, the last of the five notorious characters. Deafeat her and make your way towards the last main boss. In order to trigger her succubus form, you need to use the inspect ability during her main fight.
So, here we are. You saw the secret scenes, you defeated the 5 main succubi, the 5 notorious characters, the prostitute, the angel and our one true goddess Est (Yes, Est=best babe in the game). With all of that done, you trigger the secret "ending" of the game after defeating the 5 main succubi again. I put ending in " " because there is actually more stuff to do at this point. But I will not spoil that. Go and find out by yourselfes.