Rotom Zukan

ロトム図鑑

The Rotom Pokédex, also referred to as the Rotom Dex, is an electronic device located in Alola and serves as an upgraded version of the standard Pokédex. This particular Pokédex is considered rare, even within the Alola region.
Ash obtained the Rotom Pokédex from Professor Kukui in the episode The Guardian's Challenge!. The device remained empty until SM003, when Kukui finalized the Pokédex by having a Rotom enter it, transforming the Pokémon into the Rotom Pokédex Form. Since then, it has registered Ash as its user and accompanies him. Rotom describes itself as a self-learning Pokédex (comparable to an A.I.) and, in addition to speaking human language, it updates its database whenever it discovers new information about Pokémon. Upon encountering a new Pokémon, it displays the relevant data and has a tendency to get uncomfortably close to the creature to verify the accuracy of the information, which often aggravates the Pokémon and results in an attack. It gets along well with Ash and all his friends, but displays an aversion to Sophocles whenever he tries to "study" it.
After Rotom inhabited the Pokédex, it appears to have lost all combat capabilities and is unable to use any moves.