The option to invite Ann into the fun is the first crucial choice, inviting her will lead to the two good endings, not doing so into the two bad endings.
When you both go to confront Asteria at the adventurer's guild you have the choice to stay with her or part ways.
If you invited Ann she'll have your back and claim you, if you part ways with asteria it leads to the Loving Duo ending
Staying with her to the Loving trio ending
If you did not invite Ann, she does not have your back and if you stay it leads to the servant ending
if you go your way it leads to the independent path
Note: the game is bugged rn if you're on the "good" path it just goes to main menu when you go to meet asteria, to see what happens here you'll have to go through one of the bad endings first, once it's over it'll take you back to the fork for making the decision however it'll be the one where Ann is on your side.