The Rotom Pokédex is an enhanced Pokédex device found in Alola—and it's pretty rare even there. What makes it special is that it's actually inhabited by a Rotom, a ghost-type Pokémon that gives the whole thing sentience.
Ash got his from Professor Kukui in The Guardian's Challenge!, though it didn't become functional until a Rotom actually moved into it a couple episodes later. Once that happened, the device transformed into what fans call the Rotom Pokédex Form, and it's basically been Ash's companion ever since. The thing insists it's self-learning (basically an AI), and it can talk, understand human language, and automatically update its database whenever it encounters something new about Pokémon.
The downside? Rotom has this annoying habit of getting uncomfortably close to Pokémon to verify information, which usually just pisses them off and gets it attacked. It's friendly with Ash and the group overall, though it definitely doesn't appreciate Sophocles trying to take it apart for "research purposes."
One quirk worth noting: once Rotom moved into the Pokédex, it apparently lost the ability to battle entirely. It can't use any moves anymore, making it strictly a support device rather than a fighter.
Content compiled by AnimeList.moe from publicly available sources.
