As a wild Pokémon
When Naganadel was in its home world as a Poipole, it lived in a nest with multiple other Poipole and an elder Naganadel. A Necrozma, in its Ultra form, also lived in the same world, whose light would make the world a lush and green planet. Although it would often protect the Poipole and Naganadel, one day a meteorite came crashing down and in order to protect the world from its impact, Necrozma collided with it. But in destroying the meteorite, it turned into a different form with a black appearance and fell into a crevasse. After falling, the change in Necrozma's form caused it to be encased in a spiky and crystallized cocoon and turned the lush and bright world into a desolate and dark place. The Poipole and Naganadel would huddle together and take in the remnants of light left. Poipole later left the world with the intent of finding a better place to live.
Poipole debuted towards the end of The Professors' New Adventure!, where it was seen traversing through an Ultra Wormhole. In A Mission of Ultra Urgency!, it emerged from the wormhole on Melemele Island and started roaming around. Soon, it witnessed Ash and his classmates battling a Buzzwole during their first Ultra Guardians mission, and became fascinated by the light produced by Pikachu's attacks. It excitedly approached Pikachu while the rest of the group had their attention on whether Buzzwole's capture was successful or not.
In Love at First Twirl!, Poipole recalled Pikachu's Thunderbolt after encountering a wild Chinchou, and went looking for him, eventually finding him at the Pokémon School. While Ash and his classmates were focusing on their lesson, Poipole attracted Pikachu's attention and lured him away, wanting to see Pikachu's Electric-type moves again. After Pikachu had shared an apple with it, Poipole started showing its affection towards him with constant nuzzling. Ash's Rowlet, who was looking for Pikachu, mistook this as Pikachu being in trouble and tried to attack Poipole, only for Poipole to panic and use Toxic on Rowlet, poisoning it. After Pikachu scolded Poipole, it understood its mistake and offered to help carry Rowlet. While going through Hau'oli City, the two were attacked by Team Rocket, who tried to capture them, but were saved when Ash showed up.
Later, Poipole was brought to the Ultra Guardians base, where Lusamine contacted the group and, since Rotom had been unable to recognize it, told them the species name they had chosen to give for Poipole. Lusamine also suggested that Ash should catch Poipole and take care of it until they could figure out the location of the wormhole it had appeared from, which he agreed to. However, Poipole happily evaded all the Beast Balls that Ash tried to throw at it, thinking that Ash was playing with it. Lillie suggested that they'd demonstrate to Poipole what going into and being let out of a Poké Ball would be like, which Ash did by repeatedly recalling and sending out Rowlet. With the demonstration done and Poipole now understanding his intentions, Ash prepared to throw another Beast Ball, only for Poipole to touch the ball and be caught voluntarily, much to everyone's surprise. Later, Poipole was seen playing with Ash and his other Pokémon at Professor Kukui's house, even painting a picture of Pikachu on the wall.
In Showering the World with Love!, Ash and his classmates visited the Hokulani Observatory to witness Minior showers falling from the sky and then floating back into space. When the first Minior fell, Poipole immediately became fascinated by the shining light they emitted. One Minior, however, hadn't managed to break out of its shell like the rest, so Ash and Kiawe carried it to the observatory. At the observatory, Poipole kept examining the Minior until next morning, its shell finally broke, revealing an Orange Core. Poipole wanted to be friends with the Minior, but initially upset it by spraying some of its paint-like ooze on it. Poipole, however, understood that it had made Minior angry, and cheered it up by painting a picture of it on the wall. Over the course of the day, the two played a lot together, becoming good friends. The next night, another Minior shower came, reuniting Minior with its friends. However, as they started floating back up again, they began to fade away, as they couldn't survive too long without their shell. The same also happened to the Orange Core Minior that Poipole had become friends with, making Poipole sad, but it was calmed down by a gentle hug from Ash.
In The Shape of Love to Come!, Ash encountered Mina, an artist whose Ribombee is capable of detecting the emotions of others. With Ribombee's help, Mina deciphered that Poipole expressed its emotions through its paintings and that it loves Ash and considered him and everyone who lives at Professor Kukui's house to be its family. During the following night, Poipole had a dream about a dark place filled with multiple sad Poipole and a tired Naganadel, causing it to wake up and go look at the moon while crying. The next day, Ash noticed that Poipole was sad and met with Mina again to help find out what Poipole was feeling. Mina accepted and discovered that Poipole was on a mission of some kind. Later that day, Ash noticed Poipole painting a picture of "The Blinding One", which it had taken notice of in a book Acerola read to them previously.
In The Prism Between Light and Darkness!, Poipole returned to its home world when the Ultra Guardians followed Lunala through an Ultra Wormhole. Poipole reunited with the other Poipole and Naganadel, who told the group about Poipole's motive for coming to Alola. Poipole later joined the rest of Alola in showering Necrozma with light so it could return to its normal form. After the Ultra Guardians helped return the light to its homeland, Naganadel informed the Ultra Guardians that Poipole wanted to stay behind. Thus, Poipole left Ash and returned to its hive, but not before drawing Ash and Pikachu's faces on the ground to express its goodbye to them.
In Z-Move Showdown!, Poipole returned through an Ultra Wormhole, having evolved into a Naganadel and learned Thunderbolt since Ash last saw it. Temporarily teaming up with Professor Kukui, it helped Ash and Pikachu to attack a Shiny Guzzlord that was threatening Manalo Stadium. Together, they sent it away through the Ultra Wormhole Naganadel had just come out of, using the force of two simultaneous Gigavolt Havocs. At the beginning of the next episode, Naganadel officially rejoined Ash's team in preparation for his upcoming Full Battle against Professor Kukui.
In A Full Battle Bounty!, Ash sent Naganadel out to face Professor Kukui's Lucario during their Full Battle. The battle between the two properly commenced in the next episode, where Naganadel eventually managed to defeat its powerful opponent before being recalled. Ash later sent it back out to battle Tapu Koko, who had decided to join the battle as Kukui's final Pokémon. The battle between the two Pokémon took place in the following episode, where Naganadel, despite successfully dealing some damage on the guardian deity, was eventually defeated.
In Dreams of the Sun and Moon!, an Ultra Wormhole opened above Professor Kukui's house, and Ash, understanding that Naganadel had friends who needed it in its own world, decided to let it go. As a sign of their friendship, Ash had Pikachu use Thunderbolt for Naganadel to enjoy for one more time before it disappeared into the wormhole.
Poipole expressing itself with its drawings
As a Poipole, Naganadel is quite playful with other Pokémon. In one instance of this in A Mission of Ultra Urgency!, whilst Ash and his classmates were waiting to see if Buzzwole had been captured, Poipole attempted to play with Pikachu before flying off. It has also been shown to have a fascination with light, as shown how it tends to stop to admire Pikachu's Electric-type moves, much like how Ash's Goodra admired Pikachu's strength. In addition, Poipole is known to laugh even when attacked. Poipole took an instant liking to Ash after he helped saved it from Team Rocket. This was also seen when Ash showed it how Poké Balls work. Equally, it also became quick friends with its teammates. Despite being a playful trickster, Poipole never seems to intend any serious harm, as it clearly felt sorry when Pikachu scolded it for poisoning Rowlet, and offered to help carry Rowlet.
Naganadel had a habit of spinning the tip of its stinger when when feeling happy. As a Poipole, it span on is head instead. It also likes to express its affection for Pikachu by nuzzling him. Despite Naganadel's playfulness, it has been shown to be there for its friends, like in Some Kind of Laziness!, when it rescued and brought back Rowlet after it was almost blown away by the harsh winds blowing through the Haina Desert. Nagandel has also been shown to love making paintings with its ooze, especially of those whom it wishes to be good friends with, like Pikachu and Minior. As revealed in The Shape of Love to Come!, Nagandel uses its drawings to express its feelings to others. It draws lighting bolt-shaped marks on or next to things that it likes or loves, and has been seen drawing teardrop shapes when feeling sad.
Being an Ultra Beast, Naganadel is quite unfamiliar with the world of Ash and his classmates, and is driven by a great sense of curiosity, as seen in Real Life...Inquire Within!, when it began to play with an Alolan Exeggutor's tail head, unintentionally causing the Exeggutor to send Ash rolling out of the Pokémon Center he was in. This same curiosity almost got it in trouble with a Tauros on Kiawe's family's farm. It appears to have retained some of its playful naïveté even after evolving, such as in Exhibition Unmasked!, where it was seen watching Ash's Melmetal spin around so intently that it became dizzy, as well as enjoying being the center of attention when sent out in the middle of Manalo Stadium in A Full Battle Bounty!. As a Poipole, Naganadel appeared to lack the ability to smell, as it was unaffected by the stench produced by Garbodor in Real Life...Inquire Within! and reacted with indifference to the aroma produced by the flowers grown by Tapu Bulu in Some Kind of Laziness!.