Tales of Vesperia: Definitive Edition

To unlock all Bunny Ears you have to get all character titles in the game - minus 2. One of those will be Estelle's Itemaniac as you need The Black Bunny Ears to complete it afterwards.

(They say that titles are worth different amount of points -a hidden stat - so if that's true you could possibly unlock it missing a few more titles than 2, as long as you have unlocked all the different titles that includes new costumes already. But for me, I didn't get the ears before I had two left - Itemania and Karol's Snowboard outfit)

To join the Bunny Guild you have to talk to the man in the left corner inside the Guild HQ in Dahngrest. He's occasionally hidden behind a girl in blue.

He will give you items based on how many titles you have. Attachments for your characters like the four sets of bunny ears in the description.

The Dog Face attachment is the last gift from the Bunny Guild before the Black Bunny Ears, so then you know you are close.

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When you finally have the 4 sets of Bunny Ears they have to be worn 5 consecutive hours. And the characters can't leave the party, so it's recommended to do this in the endgame rather than to combine it with Low Level Challenger or Speedster as characters join and leaves the party constantly during the story. (They don't have to be chosen as the 4 fighters, they just have to be in the team)

So to sum up
Yuri - Black Bunny Ears
Estelle - Girly Bunny Ears
Judith - Cute Bunny Ears
Rita - Red Bunny Ears

There are other characters that also get Bunny Ears in this edition, like Patty and Flynn, but they are not necessary to wear for the achievement. Only if you want them to look cute.

Good luck getting all those titles, you can check out how to get them all at
https://gamefaqs.gamespot.com/ps4/241013-tales-of-vesperia-d...

one extra tip is to combine this with the To Points Unknown achievement. You will probably not unlock that with normal gameplay, so you might as well combine the two and rubberband Ba'ul while doing this as well.