I think you mean that some of the rare items, when used, will bring a "guest" Tamagotchi character to your Tamagotchi. They will do a little animation and then go away. For example, the book in the rare items section brings Jidoutchi for a few seconds.
There are some items that really do evolve your Tamagotchi or return them back to normal if you have a "bad-care family," but only if you have certain characters.
Large/Dainamikku Family: Balance biscuit or mirror
Petit/Puchipuchi Family: Milk or magnifying glass
Ninja/Shinobi Family: Steamed bun or whistle
Hatsugatchi -> Kuromametchi: Sesame pudding
Watatchi -> Makiko: Dresser