The Bunny Whistle is a unique item that will allow you to summon a large bunny in the world map and ride it as a mode of transportation. While riding the bunny, you’ll be able to travel the world map faster and without any random encounters.
Unfortunately, this item is missable since it is tied to Pericci, which means you’ll have to recruit her. And recruiting her requires a specific item that she drops early in the game. If you didn’t pick that item up, you won’t be able to recruit her at all.
After getting Pericci, you’ll still need to clear the castle’s Treasury as part of the story and receive the Muah Emblem if you haven’t done it yet. This is so that you can trigger a Private Action in Portmith. During the Private Action, head towards the castle to have a cutscene with Pericci.
After the cutscene, you have to walk all the way back through the Mt. Metorx’s mountain pass and perform another Private Action in Haute. Pericci is standing outside the shop so talk to her to check out the shop if her friend is there. After the scenes, you’ll receive the Bunny Whistle.
To use it, make sure you’re out in the world map then access your main menu > Items > Others.
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You forgot to mention one very important thing: in that cutscene in Portmith PA we MUST pick first choice of those two given us in dialogue with Pericci. Otherwise you just lose you chance to get this incredible item because you can't trigger the next required cutscene. I myself almost lost it lol (sorry for mess English)