Ash received the a Pokédex from Professor Kukui in The Guardian's Challenge!. It was empty until Loading the Dex!, where Kukui completed the Pokédex by having a Rotom enter the device, turning Rotom into what it calls the Pokédex Rotom version (Japanese: ロトムポケデックスフォルム Rotom Pokédex Forme). 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.
In Crystal-Clear Sleuthing!, Rotom Pokédex was shown to be a big fan of a detective show that Raki starred in and used his knowledge in an attempt to track down Ash's missing Electrium-Z. However, at the end of the episode, it has been revealed that the Z-Crystal had been stuck inside Rotom's wig the whole time.
In SM023, Rotom Pokédex updated itself with a voice duplication function. In SM024, it used this function to give Ash's speech for him during the Visitation Day, as Ash mouthed random words. The cheating was quickly caught, however, when Rotom started speaking faster, and began spitting out phrases from other people, like Principal Oak. Rotom was still happy to have given the speech.
After Rotom inhabited the Pokédex, it seems to have lost all combat functions, and is unable to use any moves. As shown in Crystal-Clear Sleuthing!, it can use the "arms" on the Pokédex to extract data from other electronic devices and now has a video camera function and a voice duplication function.
Compared to previous Pokédexes in the anime, Rotom Pokédex provides Ash with the type of Pokémon it provides data on.