I got the most yandere ending first! Took a few minutes to figure out the rest. (It also wasn't a "good" date since I was genuinely annoyed about the tattoo artist and I thought the "it's a secret" thing was flirty.) Incredible game, thank you!
Ending guide, I think? I haven't tested them after doing them ^^; Also, first thing: start the game with "options" and turn "always dash" on.
Normal endings:
Choose "you" for your meal
Have a bad date (don't share any info about yourself, be rude) and interact with enough NPCs that Serze finally finishes in the bathroom and comes to meet you
Have a non-amazing date and enter the bathroom before Serze comes back out
Have an amazing date (share all info, be nice) and interact with enough NPCs for Serze to come back out
True ending:
Have an amazing date and enter the bathroom before Serze comes back out
Hidden ending:
Have an amazing date (may not be required but I assume you at least can't be rude to the waitress), then go talk to waitress at the front counter, who will ask you to serve customers. Don't look at any other objects and don't make a mistake. (The person who came in late ISN'T the same as the diamond lady.)