Prior to its debut, Popplio was bullied by multiple Team Skull Grunts. Lana happened upon the scene and became furious at what was happening. As such, she attacked them with her Lapras's Ice Beam, freezing the group solid. She then used a fishing rod to pull it to safety. Lana then took Popplio to heal its wounds. Lana decided to personally feed and care for Popplio, as she did not want it to be afraid of humans in the future. Popplio quickly warmed up to her, and became her partner Pokémon.
Popplio first appeared in Alola to New Adventure! alongside its Trainer, fishing for wild Pokémon. Once Ash came to the Pokémon School, it helped in welcoming him. Popplio later watched very concerned as Team Skull Grunts battled Kiawe.
In The Guardian's Challenge!, Popplio was used to help welcome Ash to the school, competing in Lana's challenge against Ash and Pikachu, competing in a race. The race began on the ground where Popplio quickly fell behind compared to Pikachu's speed. Once it reached the water section of the race, Popplio immediately caught up to Pikachu, swiftly moving through the water. Popplio finished first, receiving praise from Lana and the others watching.
In Yo, Ho, Ho! Go, Popplio!, Lana had Popplio train on its water balloon, hoping to increase its size. After a few tries, Popplio managed to make one multiple times its normal size, but was unable to retain it for long, resulting with it dousing Lana and Ash. The following day, Popplio helped Lana fish for Water-type Pokémon. Soon after, the Poké Ride Lapras were stolen by Team Rocket's hot air balloon. Pikachu succeeded in breaking them free, but everyone quickly noticed that they were positioned directly over spiked rocks. Popplio swam at high speeds towards the rocks and quickly made a giant water balloon that the Lapras fell on, bouncing them away from the rocks safely into the water. It then used its water balloon to counter Jessie's Mimikyu's Shadow Ball, while Ash had his Rowlet break the hot air balloon with Leafage.
In They Might Not Be Giants!, Popplio was seen practicing on making its water balloon again, but floated away in in one of its balloons along with Ash's Rowlet when the latter accidentally bumped into it. While the others looked for the duo, Rowlet accidentally popped the balloon, causing the duo to fall straight onto a radar that Team Rocket was constructing. Popplio distracted Team Rocket by using Bubble Beam to escape, but the two were soon confronted by Meowth and James's Mareanie. A Litten then appeared who saved the duo. Litten took Popplio and Rowlet to its home, where Stoutland advised it to take the two back to their Trainers. During their long search, the trio got caught by Team Rocket. However, they managed to escape thanks to Litten and blasted Team Rocket off with their combined attacks. Afterwards, Popplio and Rowlet were reunited with their Trainers.
In Big Sky, Small Fry!, Popplio assisted Lana in the trial at Brooklet Hill by distracting a Totem School Form Wishiwashi, which Lana had hooked with her fishing rod. Although it was initially scared to battle the Small Fry Pokémon, it managed to overcome its fear after seeing Lana struggling to reel Wishiwashi in. As it started attacking Wishiwashi with Bubble Beam, Popplio soon got sent flying by an attack from an Alomomola, blasting it into a rock. After Pikachu had defeated Alomomola, Popplio attacked it with a newly learned Aqua Jet, defeating the Totem Pokémon and causing it to revert to multiple Solo Form Wishiwashi, which then made the Solo Form Wishiwashi that Lana healed earlier to hand over a Waterium Z to her.
After obtaining her Z-Crystal, Lana and Popplio had attempted to perform Hydro Vortex, as shown in Balloons, Brionne, and Belligerence!, but Popplio was unable to successfully perform the Z-Move. After witnessing Ida and her Brionne use Hydro Vortex, Lana requested Ida to train her and Popplio. Their training consisted of having Popplio attempt to burst Brionne's water balloons using its own. Despite failing initially, Popplio eventually succeeded after some hard training. Later, when the group got attacked by a wild Dhelmise, Lana and Popplio were able to successfully execute Hydro Vortex, which sent Dhelmise flying.
In When Regions Collide!, Popplio was used in a battle against Misty's Psyduck alongside Mallow's Steenee. Steenee started the battle with Magical Leaf while Popplio quickly followed with Bubble Beam. Psyduck then counterattacked with Water Gun. Popplio then created a water balloon, absorbing the Water Gun, which it then launched at the Duck Pokémon, trapping it inside. Misty commanded Psyduck to use Confusion, but Psyduck didn't respond. Steenee then attacked with another Magical Leaf, breaking the balloon and causing Psyduck land on its head, giving it a headache. This allowed Psyduck to successfully use Confusion, but before it could do anything more to its opponents, Misty told Psyduck to put Popplio and Steenee down, saying that they had already had enough.
In Rescuing the Unwilling!, Popplio was among those used to fight Lusamine's Mismagius, Milotic, and Lilligant, which were under Nihilego's control, to allow Ash to save Lusamine. Popplio knocked back Lilligant with Bubble Beam, but Lilligant then made it dance with Teeter Dance. Ash's Litten and Rowlet released it from the dance, and it rejoined the fight. In the next episode, Lana and Popplio used Hydro Vortex on Mismagius. Although Mismagius appeared to have been defeated by this move, Nihilego made it get back up and continue battling. The battle ended when Ash and Pikachu performed 10,000,000 Volt Thunderbolt to defeat Nihilego.
In Filling the Light with Darkness!, Popplio was used to help clear the mysterious black clouds that had appeared in Alola's skies. By combining its Z-Power alongside Ash's Pikachu, Kiawe's Marowak, and Gladion's Lycanroc, the Ultra Guardians powered a machine that managed to disperse the clouds.
In Securing the Future!, Popplio joined the rest of Alola in showering Necrozma with light so it could return to its normal form.
Brionne just after evolving
In We Know Where You're Going, Eevee!, Popplio spotted the Eevee that it had befriended earlier being chased by Pershie and helped protect it. Soon after they were spotted by Rapp, Zipp, and Tupp, who started fighting over Eevee. Lana and her classmates soon intervened, and during the ensuing battle, Popplio ended up learning Surf, giving Eevee a platform it could use to jump down from and send the Team Skull Grunts blasting off with a powerful Double-Edge.
In Evolving Research!, Lana and Popplio trained alongside Ida and her recently evolved Primarina. After Ida displayed Primarina's signature Z-Move, Oceanic Operetta, Lana became determined to impress her, and trained with Popplio for an entire night. The next morning, Lana showed the results of her training to Ida, which also resulted in Popplio evolving into Brionne. Before leaving, Ida gave Lana a Primarium Z so that she could one day use Oceanic Operetta as well.
In SM120, while Ash and Lana were playing with Brionne's balloons in the sea, they were unexpectedly caught in a violent whirlpool that threw them onto a rocky reef. The cause of the whirlpool turned out to be a wild Kyogre that had been poisoned and was being chased by a hunter and his underlings. Thanks to Ash and Lana's help, the hunter was defeated and Kyogre was cured of its poison. However, Kyogre was still angry, and Lana was dragged underwater by it. Brionne then evolved into Primarina and saved Lana from drowning. Seeing Kyogre's rage, Lana decided to calm it down by reeling it in and having Primarina sing to it. This plan succeeded, and Lana was able to properly greet the Legendary Pokémon she had longed to meet for a long time.
Primarina is a very happy and playful Pokémon, often seen playing and clapping in excitement. Initially afraid of people, Primarina quickly warms up to them thanks to Lana. Primarina is very close with its Trainer, as it's seen being very joyful when praised by her and was often seen in her arms or on her head, as a Popplio. As shown in They Might Not Be Giants!, Primarina is also a very responsible Pokémon, taking control as leader, when it and Rowlet were separated from their Trainers, by carrying Rowlet and even fighting against Team Rocket, even when the Rowlet's lax and sleepy behavior left it frustrated. In the same episode, it was also very apologetic after it and Rowlet crashed into Team Rocket's Secret Base and was even getting Rowlet to apologize until it realized who they were apologizing to.
Primarina tends to become jealous whenever another Pokémon was affectionate with Lana. In Dewpider Ascending!, this jealousy was showcased when a Dewpider clung affectionately to Lana's leg. In Bright Lights, Big Changes!, Lana revealed that Primarina likes to eat seaweed salad, while in That's Some Spicy Island Research!, it was shown to hate the taste of Poni Island daikon due to its spiciness. Primarina cares deeply for its close friends, who belong to its Trainer's friends, and its best friend and teammate Sandy. Primarina shows an interest in singing, possibly due to its evolutionary ties to Primarina. This was first shown in Let Sleeping Pokémon Lie!, where it took the lead with its Trainer in teaching Samson's Komala Sing. It later sung with Ida's Primarina as part of its "training". After evolving into Brionne, its singing skills improved dramatically, as seen in Keeping Your Eyes on the Ball!, and improved further when it evolved into Primarina. After evolving into Primarina, it became strong enough to swim against raging ocean currents while carrying its Trainer on its back.