Get an "S" rank on all seven missions. Then, in stage 4, save the game and choose to "Kiss on Lips ". The following dialog options must be used during your playthrough.
All are real Takashi.
Approach the princess.
Bring Takeshi forcibly to the exit door.
Cement.
Do nothing.
Don't choose.
Don't choose.
Don't sleep.
Encourage myself and follow her.
Force her out.
Isn't it hard to work in those clothes?
Kiss his lips.
Let him go first.
Make a gap between the lanterns.
Neither of the above choices is correct.
Shy.
Stop him calmly.
Walk towards the exit door.
You're the stupidest guy I've ever known.
The love ending will appear after you complete the game. You can then reload the saved game and make another choice to get the other ending.
Easy "Normal Ending Unlocked!" achievement
In stage 4, save the game, then choose to "Kiss on Forehand". The normal ending will appear after you complete the game. You can then reload the saved game and make another choice to get the other ending.