He'll appear there after doing Vince's 1st bit, make sure to sleep before gameover, and meeting everyone in town for Sep's conversation about it. Then for more content need to do Bjorn, Garius, and Vic. With Bjorn being the most important.
i bought a lot of pills, also slept before every game over, and every time i see Sep at the end it doesn't seem to have anything new for me. im using a save from the previous version, got enable the /skip unseen text to progress faster but recently realize that when i shoot that option down some game overs texts still remain like if was the first time, ill try to reset and start a new game. ( not that i mind, i always discover new things :P)
note: english isn't my native language, sorry for any misspelling
Loving your game, the best of lucks!!!
The only thing you need from Sep is his conversation when meeting every main character in the town at least once for the Vandeel requirements. The rest are progressing with Vince, Bjorn at the east gate, Garius, and Vic. Vic does require some of the potions that unlock kinks like inflation and musk.
Glad you're liking it aside from trying to figure out how to unlock characters.