Rotom first appeared in Robust Rotom, when Diamond and Pearl wandered into the Old Chateau. After watching the two do their comedy act Rotom decided to play with them by attacking them in its plasma state. Pearl has his Chatler and Chimler attack Rotom, who simply jumps around between electrical appliances to avoid the attacks. Diamond quickly figured out the last place Rotom would go to was the TV since it was its home and waited for it there. After Rotom appeared he had Lax and Tru ram the TV to get Rotom to settle down. Diamond and Pearl left the next day without figuring out what exactly the Plasma Pokémon was.
In Unplugging Rotom (Heat, Wash, Mow, Fan, Frost), Rotom was one of the Pokémon Charon was interested in. Due to Charon wanting to capture it, Professor Rowan decided to send Diamond and Pearl to protect Rotom since they had already had an encounter with it before. Diamond and Pearl find Rotom inside the TV, and Rotom quickly exits it and reveals its true form. Diamond and Pearl both attack Rotom with their Pokémon, but dealt no damage due to it being an Electric/Ghost type and having the Ability Levitate. Rotom then moved towards Diamond and Pearl and shocked them while giggling before leaving for Eterna City.
When the two find Rotom, it leads them into a secret room, which has five electrical appliances in it. Rotom appears from the ceiling and jumps into the washing machine. The washing machine takes on a similar appearance to Rotom and begins attacking them with Hydro Pump. Diamond brings out Tru to try and counter it, but Rotom simply moves to the microwave oven and attacks them with Overheat. Pearl quickly realizes that since Rotom is made out of plasma, it can possess appliances and gain special moves based on the appliance it's possessing. Rotom moves into the fan next, turning it into a Flying-type and allowing it to blow the two away with Air Slash, which causes the group to get blown outside. Rotom moves to the fridge next, turning into Frost Rotom and attacking with Blizzard. Rotom lastly possesses the lawn mower next, and chases the two alongside the grass. Pearl decides that Rotom will calm down if it returns to its TV and leaves Diamond with Rotom in order to get it.
Rotom possessing a Proteam Omega
When Pearl returns he finds Diamond and Rotom lying in the grass relaxing. Diamond reveals that Rotom was not actually trying to attack them, but was just playing around. After Rotom stopped chasing him, Rotom cut the grass to look like Diamond's face, showing that it was friendly. Diamond reveals that while Pearl was gone, Rotom used its forms to make snacks and wash and clean his clothes until they eventually fell asleep on the grass. The two decide that Rotom is powerful enough to protect itself and start to leave. Rotom then tells them that it wants to stay with them, and joins their group.
In The Final Dimensional Duel XI, Rotom possessed a Proteam Omega action figure and destroyed Charon's machine in order to stop him from controlling other Pokémon. Rotom then uses Thunder Shock, causing Charon to pass out. With Charon defeated, Rotom decided it wanted to stay with Diamond longer and joined his group.
Personality and characteristics
Rotom and Diamond
Rotom is a mischievous prankster and often attacks others to entertain itself. It is a very useful Pokémon due to all of the form changes it has, and is considered powerful enough to defend itself from even Charon. Rotom is very attached to its Trainer, shown when it wanted to follow him and when it cried at Diamond's supposed death. They both share a love of robot anime.