This felt like it would take a bit too much space to have in details for the Achievement guide, but here’s a step by step guide to get one of the harder achievements of the game. This is not the only way to get the achievement, but it’s guaranteed to work.

There are more conversations with Vonne than the ones listed, but they shouldn't be relevant to the achievement, so you can play them out as you like.

Since you’re going to lose at least two minions, you may as well try for the Minimalism achievement too. This means you should start the game with either Althea or Thaïs as the traitor. Once you’ve completed the game, you can start a new game where you choose the traitor from the onset. I suggest you pick Althea.

Until October 24th, you should raise Vonne and Thaïs’ loyalty to the maximum of 45. Your extra loyalty points should be given to Rowan, since you’ll have to rely on her for a while. Invest your skill points in getting Rowan’s Stealth and Observation high, and raise Thaïs’ Charm and Intimidation.

If you don't care about the Minimalism achievement right now, you can just follow the dialogue choice guide, choose whichever traitor suits you best, and raise whatever stats you prefer. You should know that if Mallory is kicked out, Vonne will take over her Social stat, and if Rowan is kicked out, Vonne will take over her Observation and Stealth stats, so plan accordingly.

On October 24th, finger the traitor, Althea, and choose Vonne for your Queen.

November 5th, go to the carnival with Vonne. When the choice comes up, say:
“The teachers forced me to”
“They thought you were ‘too attached’”

November 11th, say:
“We need to talk”
“I wanted to apologise to you”
“I should have told you how I felt”
“I like you”

November 12th, say:
“What’s the Silver Cat?”
“I’m not interested”
“You’re overreacting”
“You should have asked first”
“Shall we kiss and make up?”

November 14th, say:
“Your heart belonged to me”
“Do you want to?”
“(Kiss her)”

December 6th, say:
“Am I interrupting?”
“What was that all about?”
“It sounded like more than that”
“Is that all?”

January 6th, say:
“What sort of game?”
“I don’t know about this”
“If you trusted me, you would tell me”
“All right”

January 9th, say:
“We’ll keep our games private”

January 16th, say:
“What do you want me to say?”
“I want to keep this secret”
“The others might be jealous”
“Secret meetings are romantic”

February 12th, say:
“(Pull away)”

On February 17th, you get the achievement, The Yandere Principle.

Now, if you can finish the game and defeat the final boss with just Vonne and Thaïs, you will also get the Minimalism achievement. Nobody said it would be easy though.