Ever since his debut, Sandy shows himself to be curious, naive, yet upbeat, frequently greeting others around his amiably or sniffing in an inquisitive manner. However, he can also come off as somewhat inconsiderate, such as unintentionally scaring a Wimpod while chasing it and causing a Pyukumuku to punch him. He is also quite gluttonous, as he always gets exhilarated upon seeing any type of food.
Sandy cares deeply for his friends and would go out of his way for them, as shown when Sandy's good friend Mantine, along with other Mantine, were captured by Team Rocket. Sandy also developed a close friendship with Shaymin, Snowy and Ash's Pikachu. Much like Primarina as a Popplio, Sandy was also saved from Team Skull by Lana. Sandy shares a close bond with Lana and is very loyal to her. In addition, Sandy is close to his Trainer's family, especially Harper and Sarah.
Once a nameless Eevee, Sandy was walking along happily by the docks until he spotted a bowl of food. Sandy eagerly devoured the food until realizing the food bowl belonged to an angry Houndour, which began to chase the surprised Eevee. After managing to lose the Darkness Pokémon in a crowd of people, Sandy stood on a boat, brushing his bangs away from his eyes and marveled at the view of the ocean.
When a Wailord shook the ship, Sandy was revealed to be sleeping inside a crate, which fell into the sea. Upon waking up, Sandy began to panic and started to drown after falling from the now sinking crate he was on, but was rescued by a wild Mantine, who soon took Sandy to a safer destination. As Sandy and Mantine approached the land, they had a little fun playing on the waves, where they passed by a boat holding Lana’s father. Eventually, they reached Melemele Island where Sandy was dropped off on the beach, as they bid each other farewell. Sandy soon noticed and chased a Wimpod for an attempt to talk with him, but the Wimpod was terrifed by his playful Tackle. Although unable to fit himself into the cave where Wimpod hid, Sandy noticed some Nanab Berries on the top of the island’s forest trees, above the beach, simply by pure luck.
After waking up and eating Berries for breakfast, Sandy over-enthusiastically greeted a nearby Pyukumuku, but was then hit with an uppercut by the scared Pokémon. However, water balloons caught Sandy in the air and the Pyukumuku who punched him. Sandy found the source of these balloons to be from Lana's Popplio, who was playing with Ash's Pikachu and the rest of their Alolan Pokémon friends, and finally got the chance to meet them. While swimming with Popplio, Sandy's swimming was revealed to be improved, thanks to its experiences with Mantine, and Popplio was impressed. However, while playing with its newfound friends, Sandy accidentally hit one of Turtonator's back spikes, which then exploded. Although only Sandy, Pikachu, Sophocles' Togedemaru and Kiawe's Marowak got hit, Marowak's scolding of Sandy was cut short when he, Marowak and their friends noticed and laughed that Pikachu was revealed to only have a head-burn, instead of a full body burn. Sandy later brought the fruits from his tree to have lunch with his friends. In the afternoon, when his friends needed to return to their Trainers, Sandy bid them goodbye and promised to meet again somewhere.Sandy was eating some berries in his cave before trying to make friends with a Furfrou, which got annoyed by him. After the Furfrou left with its Trainer, Sandy thought about his new friends and decided to find them. While looking through town, Sandy saw Popplio in the arms of her trainer, Lana, then followed them into the store they were going to. Sandy greeted a Persian, who had caused some trouble for his friends and their trainers in the past. Believing it to be a new friend, Sandy soon realized that it did not want to be his friend when Persian attacked him, causing him to run away. Sandy being chased was notices by Popplio, as she and her trainer were getting some malasadas for a dinner they were having with her trainers family, as she left her trainer to help him. Hiding in a bush, Sandy was found by Popplio as he was filled with joy to see one of his new friends again. However, Sandy unknowingly left his tail hanging out of in the bush they were hiding in, which the Persian noticed. When discovered by Persian, Sandy and Popplio ran as fast as they could, escaping with Popplio's Aqua Jet. Sandy and Popplio landed in the stream, and were pleased to know that they were no longer being chased, and then swam away. After ending up on the beach in the outskirts of the city, Sandy discovered some Pokémon belonging to Team Skull, angering them by using Tackle to greet them. After Team Skull showed up to check on their Pokémon, Sandy was pulled around by Rapp, who wanted the Evolution Pokémon for herself, and Tupp, who wanted an Eevee to make him popular with girls. However, Sandy was saved when Lana, Lillie, Mallow, Sophocles and Ash, who had been looking for Popplio, showed up, as Pikachu used Quick Attack to get the Team Skull Grunts to let go of the Eevee, with Popplio catching him before he fell into the sand. Amazed by his best friend Trainer compassion and remembering her from before, Sandy joined Popplio in greeting Lana, then Tackled his other new friends, minus Turtonator and Marowak, in affection, as he was happy to see them again, surprising their trainers that the Eevee knew their Pokémon. While Rapp and Tupp still argued about who got to keep him, Sandy was grabbed by Lana and witnessed Ash yelling at them that they were not giving him to either of the grunts because of their behavior. Seeing that Popplio and Lana were determined to protect him, Sandy was inspired to battle Team Skull alongside them, jumping onto some Water-Type Pokémon to get on top of the waves created by Popplio's newly learned Surf then used Double-Edge to send the Team Skull Grunts blasting off. Sandy was praised by Popplio and their friends for his efforts. After Kiawe, who was also looking for Popplio, arrived at the site, Sandy and Popplio were praised by Lana for their teamwork as they played in the water together. Sandy happily accepts Lana's offer to go with her as he already bonded with her and knowing he’ll be with his best friend as teammates. After being caught in a Dive Ball, Sandy was brought to his new home as he was introduce to Lana's family as her newfound friend and partner, then was given a haircut to help him see better.
Later, Sandy was introduced to Misty and Brock, who were visiting Alola. After Brock offer to stay and help with Nurse Joy, Sandy went with everyone as they showed Misty around. While Mantine Surfing to Treasure Island, Sandy got to see his friend Mantine again and showed Lana along with their friends on how to Mantine surf. Sandy was having fun with his friends on Treasure Island but Team Rocket showed up and stole the Mantine. However, Sandy with his friends were able to save the Mantine. Also, Sandy accompanied Lana as she and everyone showed their friends Akala Island. While Brock and Olivia were having a battle, Sandy played with its friends until Team Rocket showed up and captured them, but they were saved by Brock and Olivia.
Sandy headed over to Poni Island with Lana and Popplio along with their friends, who were doing their research projects over on the Island. Hearing his Trainer's project plans, Sandy was very excited to meet his Trainer and best friend's mentors, Ida and her Brionne. While waiting for Ida's arrival, Sandy swam with Popplio.
When Ida finally arrived on Poni Island, Sandy met Lana's mentor, along with meeting Popplio's friend and mentor Primarina, who had just evolved from Brionne. He joined in on the training with Popplio and Primarina. Later at night, Sandy joined his trainer to watch her and Popplio practice. In the morning, before Ida left, Sandy watched his best friend Popplio blew a giant bubble balloon, as she evolved into Brionne. Sandy was with Lana and Brionne at the beach doing some training before they all decided to take a nap on the beach.  While at Hapu's house, Sandy and Lana were helping at Hapu's radish fields while some of their friends became lost in a fog that was created by Tapu Fini. Sandy and Lana with their friends, Sophocles and Kiawe, waited at Hapu's house until the fog cleared up. After the fog cleared up, Sandy formed a close friendship with the Mythical Pokémon Shaymin that Mallow found. Afterwards, Sandy watched Ash prepare for his grand trial and later rejoined Lana when returning.
Sandy later participated in a movie with its trainer.
After arriving with its friends at windy Malie City, Sandy and Meltan begin to cause trouble after seeing a line of Eevee, much to Lana’s chagrin. When Shaymin tries to get Mallow’s purchased bandanna that was blown away by a powerful wind, Meltan and Sandy followed and helped it. Though Team Rocket ambushed them afterwards, Meltan freed them from Team Rocket's trap by eating the metal bars, but Meltan, Shaymin and Sandy were attacked upon escaping, so both Sandy and Meltan fought Team Rocket off while protecting Shaymin until Ash, Mallow and Lana arrived. While Shaymin healed Sandy who got hit by Sludge Bomb from James' Mareanie, Meltan used a new move, Headbutt, immune to Mareanie’s Poison-type attack, with Sandy performed the final blow before Stufful and Bewear dragged Team Rocket away.
Sandy later watched Ash and Kiawe's battle against the Gym Leader of the Kantonian Gym, Ryuki. While Kiawe lost the first part of the battle with his Marowak, Ash managed to defeat Ryuki in a double battle with Meltan and Rowlet.
Sometime later, Sandy watched Sophocles and his Vikavolt participate in the Vikavolt Race, which they won and received the Buginium Z. After the race, Sandy witnessed Hapu giving Sophocles a Z-Ring for his valiant efforts of saving the other contestants' Vikavolt from Team Rocket.
Sandy later assisted Lana in rescuing a Legendary Pokemon Kyogre from an illegal group of hunters. And after successfully driving the villains away, it witnessed Brionne's evolution into Primarina. It then supported Lana and Primarina in their efforts of reeling Kyogre and witnessed Primarina soothing it with her singing.
When Mallow was working at Forest Pokémon Café to help Oranguru serve hungry Pokémon, Sandy and Primarina helped Lana as extra waiters in Aina's Kitchen. Due to the countless customers who had come to dine, all three of them were worn out by the time Mallow returned. Also, Sandy and Primarina were excited that their Trainers all have Z-Rings.
At the School along with the others, Sandy watched its best friend Primarina and their Trainer perform a new Z-Move Oceanic Operetta. At Lillie's Mansion, Sandy was then seen having lunch with its friends. Sometime later, Sandy and his friends were training for the Alola League at Lillie's house. 
At the Alola League, Sandy was used in a Battle Royal during the first round and one of 16 competitors to move to the finals.  Sandy comforted his best friend Primarina after she lost her battle with Guzma's Golisopod.  During the semi-finals, Sandy and his best friend Primarina was then watching Ash battle with Guzma.
At some point during the Ultra Beast crisis, Sandy helped join everyone to defeated one of the Guzzlord at the Manalo Conference. 
- Before its actual capture, a promotional poster and the first opening theme featuring Sandy strongly implied that it was to be captured by Ash due to Sandy being seen in both shots near the trainer until Lana captured the Eevee instead.
- The poster was later updated to reflect on his new hairstyle, but it's still next to Ash.
- Sandy being owned by Lana was foreshadowed by his interactions and experience with Mantine and other Water-typed Pokémon.
- Sandy and Snowy are the only main nicknamed Pokémon in Sun & Moon.
- The name Nagisa means calm beach in Japanese.
- Sandy is the first Pokémon to be caught in a Dive Ball in the anime.
- His capture also marks the longest duration between one of Ash's female friends' first and second captures, taking a 99 episode gap.
- Sandy can be somewhat similar to Ash's Totodile, in terms of both being upbeat Pokémon who were caught in special Poké Balls.
- Sandy is the third Eevee to be caught by a female traveling companion of Ash's. However, he is so far the only one of them to have not evolved, the first Eeveelution being May's Glaceon and the second being Serena's Sylveon.
- Sandy is one of the known expanded Pokémon of Ultra Guardian, who is not one of its original founding Pokémon, the others are Naganadel, Shaymin and Melmetal.
- ^ SM099: We Know Where You're Going, Eevee!
- ^ SM102: Alola, Alola!
- ^ SM103: Heart of Fire! Heart of Stone!
- ^ SM104: That's Some Spicy Island Research!
- ^ SM105: Showdown on Poni Island!
- ^ SM106: Evolving Research!
- ^ SM107: Run, Heroes, Run!
- ^ SM108: Memories in the Mist!
- ^ SM109: A Grand Debut!
- ^ SM113: This Magik Moment!
- ^ SM114: Beauty is Only Crystal Deep!
- ^ SM117: Drawn with the Wind!
- ^ SM118: Aiming for the Top Floor!
- ^ SM119: A High-Speed Awakening!
- ^ SM120: The One that Didn't Get Away!
- ^ SM121: A Recipe for Success!
- ^ SM123: A Fiery Training Camp Trick!
- ^ SM127: Chasing Memories, Creating Dreams!
- ^ SM129: Battle Royal 151
- ^ SM134: The Road to the Semifinals!
- ^ SM136: Getting Down to The Ire!
- ^ SM140: Z-Move Showdown!