The Rotom Pokédex (Japanese: ロトム図鑑Rotom illustrated encyclopedia), commonly referred to simply as Rotom Dex (Japanese: ロトムRotom), is a main Pokémon character in the anime and Ash's Pokédex in the Sun & Moon series.
An empty Rotom Pokédex
Rotom debuted in Loading the Dex!, where it entered the empty Pokédex Ash received earlier from Professor Kukui, turning itself into the Pokédex Rotom version (Japanese: ロトムポケデックスフォルムRotom Pokédex Forme). After introducing itself to Ash, it followed him to the Pokémon School, where it met his classmates and Samson Oak. Later, it accompanied Ash and his friends during their fieldwork in a forest.
In Crystal-Clear Sleuthing!, Rotom was shown to be a big fan of the detective show Alolan Detective Laki and impersonated Laki throughout the episode. Upon finding out that Ash had lost his Electrium Z, Rotom decided solve the case. After learning about a camera crew that had been filming at the Pokémon School, it went to the Alola TV station, where it copied the video from one of the cameras and updated itself with a video camera function. Using its new function, Rotom concluded that Kiawe was the culprit, but was proven wrong by Lillie. It was then revealed that the Z-Crystal had been stuck inside Rotom's wig the whole time. At the end of the episode, Rotom offered to help Ash and Kiawe with their training by recording it.
In The Dex Can't Help It!, Rotom left the Pokédex after getting hit by Ash's Pikachu's electricity. Afterwards, it was swapped with a Wash Rotom, whereupon it made multiple attempts to get back to the Pokédex. In the end, Rotom finally managed to return to Pokédex after getting hit by Pikachu's Thunderbolt again.
In Bright Lights, Big Changes!, while observing filming of the latest episode of Alolan Detective Laki, Rotom interrupted the production after discovering an error in the plot. Rotom impressed the producer George Charino with its knowledge and was offered a role on the program. In the end, however, Rotom went back to Ash.
Rotom claims to be a self-learning Pokédex (similar to an A.I.) and has the ability to speak human language. It has a bad habit of getting up close and personal to the Pokémon in order to see if the information about it is true, only to annoy and get attacked by said Pokémon. It also updates itself whenever it meets a new Pokémon by taking pictures of it or when it finds something new about a Pokémon. As shown in Rising from the Ruins!, once Rotom Dex updates its photographic database and finds a way to describe the new Pokémon, the updated information then becomes available to download on various other Pokédexes. Rotom gets along with Ash and all of his friends, but has shown an aversion to Sophocles's attempts to "study" it. Largely due its self-learning nature, it also has an innate desire to catalog any Pokémon within the region. In Alola, Kanto!, it left during a field trip to Kanto on its own to try and catalog all 151 Kanto Pokémon, only missing one, Mew, by the time it returned to Oak's Lab.
While inhabiting the Pokédex, Rotom does not appear to have combat functions, and is unable to use any moves. As shown in Crystal-Clear Sleuthing!, it can use its "arms" to extract data from other electronic devices and update itself with the device's function. Additionally, if it is upside down or sideways, it can recalibrate its screen to show the data the right side up. Rotom also uses the numbers on its screen to express the statistics, usually showing the percentage of something, for example, to show the likelihood of a Pokémon appearing nearby. As shown in Let Sleeping Pokémon Lie!, Rotom has a clear waterproof casing, allowing it to be used underwater.
Rotom is also a huge fan of the show Alolan Detective Laki. It likes to cosplay as the titular character Laki, wearing a wig modeled after his hairstyle of Laki, which Professor Kukui bought for it, to achieve this. It also looks up to Laki and even used his thought process in order to figure out how to deal with the fact that it had become frozen. However, Rotom tends to jump to conclusions with its deductions, such as in Crystal-Clear Sleuthing!, when it believed that Kiawe stole Ash's Electrium Z. This was also shown in Lulled to La-La Land!, when it accused Lillie of putting Ash and the rest of the group to sleep and draining their energy, causing Mallow to jump to her defense. In addition, it is known to state the obvious through its calculations. Rotom is prideful in expressing the information it helps to deliver. So much so that in Bright Lights, Big Changes!, it was revealed to have realized its goal of becoming the universe's greatest Pokédex.
In The Dex Can't Help It!, it is revealed that if Rotom takes too much of a shock, it will be forced out of the electronic it is inhabiting. As mentioned in the episode's Poké Problem segment, Rotom has spent so much time in its Pokédex form that it feels uncomfortable outside of it.