Upon finishing Rule of Rose with the Good Ending, Jennifer will receive the Four-Leaf Key, which unlocks the Four-Leaf Clover Room on the Airship. Once accessed, Jennifer and Brown can wear multiple costumes.
Some costumes have certain requirements in order to be unlocked. All costumes come with special "Master" weapons which show up in the inventory after wearing the costume. The Master weapons are basically the strongest weapons in the game, along with the Rusty Sword/Knight Rapier. The Master weapons can also be used with different costumes. For example, default Jennifer can wield Master Tuna, octopus Jennifer can wield Master Racket, etc.
Jennifer and Brown can turn back into their default costumes by pressing X while Jennifer is facing the mirror in the Four-Leaf Clover Room.
After "The Unlucky Clover Field" chapter, the room is only accessible again in the "Bird of Happiness" chapter, so this chapter is the last opportunity to change it for the remainder of the game. The Four-Leaf Clover Room locks itself so the player must use the Four-Leaf Key to unlock it again.
Brown as a Chair: By observing the wooden chair along the back wall in the Four-Leaf Clover Room, Brown will turn into said chair. He will still act like his normal self, and his chair form will bend as he walks and moves around. If Jennifer pets Brown as a chair, she will do so as if he is still a dog and touches his "invisible" head.
Tennis Instructor: Obtained by getting the Good Ending. Jennifer wears a tennis outfit, which includes a white shirt, a red blazer with yellow detailing, a green plaid skirt, black kneesocks and brown shoes. Brown doesn't change with this outfit. It comes with the Master Racket.
Nurse Outfit: Obtained by getting the Good Ending. Jennifer wears a white nurse's uniform, with matching dress, hat, socks and shoes. Brown doesn't change in this outfit. It comes with the Master Syringe.
Gothic Lolita: Obtained by getting the Good Ending. This black lolita dress with white frills, with matching hat and footwear, also changes Jennifer's hair, so it is now dark brunette, shoulder-long and straightened. Brown gains a similar outfit, with black fur. It comes with the Master Umbrella.
Octopus and Crab: Observe the October Ticket in the Cell of Remorse in the "Once Upon a Time" chapter, and Jennifer can wear an octopus outfit. Brown turns into a large crab. It also comes with the Master Tuna.
Rag Doll and Stuffed Toy: To gain this outfit, find four Broken Robots in the "Rag Princess Sews" chapter. Jennifer and Brown turn into the rag dolls that appear during the game. It comes with the Master Sword.