Meloetta originally lived among Ridley's ancestors in the Abyssal Ruins, watching over the Reveal Glass. They often had to fend off others who sought the power of the Reveal Glass for themselves, with the attacks growing increasingly frequent over time. Eventually, Meloetta sealed the Ruins—along with the Reveal Glass—and fled with the guardian tribe to the forest, where they resettled and lived in peace.
For many years, Meloetta resided in the village of the forest people, eventually befriending Ridley as he grew up. However, it was eventually attacked and captured by Team Rocket. Ridley tried to stop them, but before he could do so, Team Rocket escaped in Dr. Zager's helicopter. Meloetta managed to escape from Team Rocket's captivity by disappearing, forcing the trio to begin searching for it.
Team Rocket's first attempt to re-capture it on-screen was in Virbank City in the episode An Epic Defense Force!. It was able to elude them but somehow made its way into the Pokéstar Studios studios where Ash had unknowingly encountered it. After he saved it from a collapse of some tools it stayed close behind him, and started to follow him, first to the Virbank Gym.
It reappeared in Rocking the Virbank Gym! Part 1 and Rocking the Virbank Gym! Part 2 where it watched Ash's battle against Roxie and was visibly pleased to see him win.
Meloetta appeared again in All for the Love of Meloetta! where it was again pursued and attacked by Team Rocket. Jessie had Woobat attack it with Air Slash, and James had Yamask and Shadow Ball, respectively. Meowth then tried to catch it in an electric net but once again, Meloetta escaped by invisibility. However, it still managed to get injured in the process and wandered into the street in a daze, and almost got run over by Cynthia's car. Although Ash had only seen it briefly before, he was able to recognize it and Cynthia applied Potion to its wounds. Then they attempted to get its fever down with a bag of ice they had previously smashed from a nearby lake. While Iris and Cilan searched for an Oran Berry for it, Ash kept watch.
After a little bit Oshawott came out of his Poké Ball sensing Meloetta's presence and instantly fell in love with it. Meloetta recovered a bit after eating an Oran Berry Emolga brought. During this time both Emolga and Oshawott were arguing with each other and Meloetta began to sing in order to calm them down. Afterwards, it disappeared once more but had followed Ash and the others to Cynthia's jet.
In the following episode, Meloetta appeared once again while Jervis was taking them to Cynthia's villa. Although it was reluctant to show itself to Dawn at first, it eventually did with encouragement from Ash. Immediately, Dawn's Piplup fell in love with it too. This angered Oshawott, and caused it and Piplup to constantly fight over Meloetta, and get angry when it chose one over the other.
In Expedition to Onix Island!, it interacted with all of the other Pokémon while at the beach. Later, it tried to soothe a group of wild Onix that were angry at Ash and his friends by singing them a song. However, when that did not work, it immediately changed into its Pirouette Forme for the first time since joining Ash and leaped into battle.
In Meloetta and the Undersea Temple!, Meloetta battled Krookodile after wishing to do so. However, they were interrupted by Ridley and his Golurk, who mistakenly thought the battle was against Meloetta's will. It was soon attacked and captured by Team Rocket, along with Ash and Pikachu, who were also captured. Subsequently, all three were taken to the Abyssal Ruins.
In the following episode, using the Reveal Glass, Giovanni summoned the Forces of Nature to fight Ash, Iris, Cilan, Ridley and Cynthia. After various failed attempts, Meloetta managed to calm the trio down with its song. After Team Rocket retreated back to Kanto, Meloetta reunited with Ridley at last, and accompanied him back to the village, much to Oshawott's dismay.
Meloetta dancing with Piplup
Normally, Meloetta is an extremely shy Pokémon and will hide from strangers because of its tragic past. However, it will quickly warm up to someone new and is willing to help them if needed. Though a genderless Pokémon in the games, Meloetta displays many feminine traits. It seems to like Ash most of all, at times exhibiting signs of a crush on him, while Oshawott and Piplup like it. While it's timid in its Aria Forme, it becomes a strong battler after transforming into its Pirouette Forme and can take on opponents much larger than it is.
Meloetta can use its voice to calm down the Pokémon around it if they happen to be irritated, which has a similar effect to Oración. Besides singing, it likes to dance when it is feeling happy, as seen in Piplup, Pansage, and a Meeting of the Times!, when it became so impressed by Piplup's moves that it interrupted his battle with Pansage to dance with him. It often twirls when floating around.
It can turn invisible, but it can be seen in this state if proper equipment is used, such as the kind Team Rocket has. It also possesses a song that will unseal the Abyssal Ruins, which it restrained itself from singing. Following the interaction with Ash, it let its guard down and eventually sang the whole song on a seashore which was recorded by Team Rocket. It also has the ability to channel the energy required to control the Forces of Nature while they are in Therian Forme. However, this will cause Meloetta a lot of pain.