Rotom Zukan

The Rotom Pokédex, also known as the Rotom Dex, is an electronic device found in Alola and an enhanced version of the Pokédex. This special Pokédex is said to be rare, even in the Alola region.

Ash received the Rotom Pokédex from Professor Kukui in The Guardian's Challenge!. It was empty until SM003, where Kukui completed the Pokédex by having a Rotom enter the device, turning Rotom into the Rotom Pokédex Form. It has since registered Ash as its user and follows him around. Rotom claims to be a self-learning Pokédex (similar to an A.I.) and along with its ability to talk human language, it also updates itself whenever it finds something new about Pokémon. After finding a new Pokémon, it will display its info, and has a bad habit of getting up close and personal to the Pokémon in order to see if the information is true, only to aggravate and get attacked by said Pokémon. It gets along with Ash and all his friends, but has shown an aversion to Sophocles whenever he attempts to "study" it.

After Rotom inhabited the Pokédex, it seems to have lost all combat functions, and is unable to use any moves.

(Source: Bulbapedia)