Rotom is very energetic and active, and it has a habit of explaining Pokémon to Ash and his friends. Also, it frequently updates its data of the dex. Unluckily, its boldness can cause the Pokémon that it observes get angry and attack it.
Rotom Pokédex first debuted as a normal Rotom. It fused with Ash's Alola Pokédex to become one. At first, it wanted Ash to call it Rotom Pokédex Form but it then accepted Ash calling it Rotom after it saw Ash's got tangled into a knot.
In SM042, Rotom became excited when it takes pictures of all the Kanto Pokémon at Professor Oak's laboratory and to record everything. As everyone are doing the welcoming party by sunset, Rotom finally completes its record of the Pokémon in the Kanto region, but it soon becomes incomplete due to Mew's presence who is hiding behind the bush to observe Ash and the others before it happily leaves.
None of Rotom's moves are known.