Fess/Lust Friend

Fess (フェッセ Fesse) is a boss in Lust Friend. She can only be fought after first experiencing the normal ending. You may fight her by sleeping in the bed in Fess Space (for any period of time) and, when a choice appears, pick the first of the two options.

Strategy
One should aim to fight Fess during the day, since she (along with all other enemies) does less damage during the day than during the night. If it's currently nighttime, you can skip to day without fighting Fess by sleeping in the bed and then picking the second option to avoid the fight.

To befriend Fess, pick any of the options "What do you like?", "Thanks for helping me" (both in Talk), "Touch her body" (in Action), "Impersonate Fess" (in Joke) and "Beg for mercy" (in Pillow Talk). After picking any combination of these five times (you can even pick the same option five times), you will obtain the Friend Flag. Alternatively, you can get the Friend Flag in a single turn by using Gift and giving her a Love Pendant, should you have one.

If you wish to defeat Fess, she is weak to Ass attacks. Note that she has two holds, one Chest element and the other Pussy element.