Rotom first appeared in Secret of the Chateau! It watched as Lewis and his friends entered the Old Chateau in order to take shelter from a storm. Deciding to prank its visitors, Rotom began using its different forms to chase the others around, displaying the unique ability to change its forms at will.
When the rain stopped, the group decided to leave the house, saddening Rotom, who had enjoyed their company. When Rotom flew off, the group followed it to the basement of the Chateau, where they discovered an abandoned laboratory filled with electrical appliances. There, they discovered that Rotom was the subject of a scientist's research, who subjected Rotom to several painful experiments in order to force it to be able to change its form without the need for an appliance. Horrified at this revelation, Lewis realised that Rotom pulled pranks in order to make friends and offered it a spot on his team. Overjoyed, Rotom accepted and Lewis caught it in a Poké Ball. Immediately after being caught, Rotom popped out of its Ball and began pulling pranks with its new friends as they continued to Eterna City.
Rotom is a very hyperactive Pokémon, often racing around and refusing to stay still. It has a habit of pranking others, which was revealed to be its way of making friends, and often laughed as it did so. Rotom took a great liking to Lewis, who would often become the target of Rotom's shocks. Rotom always pops out of its Poké Ball when it wishes to do things, so much so that Lewis only has to mention Rotom's name for it to come out.
Behind Rotoms' fun-loving nature, lies a much sadder story. Rotom was previously subjected to several painful experiments by a scientist and, as a result, is able to switch between its forms without the need for an electrical appliance. Rotom seems to hold a grudge against the scientist that did this, but being with Lewis showed it that it can use this ability in battle to great effect.