Much like Ash's Noivern when it was a Noibat, Nebby unleashes a high pitch scream when it gets upset. Nebby is also very friendly to those around it and is very playful when using Teleport on its friends with a smile on its face. It is also very picky about food and only eats star-shaped candies. It sometimes Teleports away just to get its favorite food. As Nebby evolved to its final evolutions, it still retains it likeness for the star-shaped candies.
Nebby has a close bond with Ash seeing him as a parental figure and their closeness is seen when Nebby uses it's powers to teleport back to him on it's own. It even has a close friendship with Pikachu and Rowlet as it spends the most time with them as it likes to sleep with both of them although it takes turns on who to sleep with also it likes playing with them as well.
Also, Nebby even took a liking to Lillie even forming a strong friendship with her as well. Nebby went to great lengths to help Lillie, shown when it answered her desire to see her brother and when it tried to teleport them away from danger. Also, Nebby even grant Lillie's wish when she wanted to remember her past that were related as to why she had her fear of touching Pokémon.
It is also very curious when reading the thoughts of others teleporting Ash and his friends to various places wishing to see the outside world even though if it's aim is off. In the end of Now You See Them, Now You Don't!, Nebby was revealed to be mischievous as it cries and teleports everyone at the top pillar of the Pokémon school to make them horrified on Nebby's reading from their thoughts.
Nebby is also helpful at times towards its friends. After becoming Solgaleo, Nebby became a strong, heroic, and independent Pokémon as it was willing to help Ash and the gang rescue Lusamine from Ultra Space. Also, Nebby became very protective towards anyone its cares about, shown with Lunala and Ash, and would do anything to keep them safe from danger.
Nebby first appeared in Ash's dream where he witnessed it being created by Solgaleo and Lunala. Then, on his way to the Pokémon School Ash and Pikachu see Tapu Koko who led them to Cosmog and they found it sleeping. Once Ash arrived at the Pokémon School, he showed his classmates the little Pokémon and they are amazed at the discovery. However, when it woke up, it let out a high-pitched scream whenever it cried. After calming it down, Lillie decided to name it Nebby while Ash decided to take care of it as he made a promise to Solgaleo and Lunala to take care of it.
Nebby was revealed to have learned Teleport and teleported Ash and his friends to various places. But also causes mischief in the end when it cries and teleports them into the top pillar of the Pokémon school much to their scare. 
Nebby comes with Ash and the gang as they visit the Aether Foundation, Faba wishes to see Cosmog but as he tries to come closer it teleports away. As the kids were enjoying the food, Nebby tried to eat a big bowl of sugar stars until it was stopped by Ash before it gets a stomach ache. Then, it teleports all over the place taking sugar stars from Team Rocket and teleports back to Ash before the trio can catch it.
Nebby teleported Ash, Pikachu and Lycanroc to Gladion's location where they came across him and his Type: Null. This frightened Nebby, causing it to cry until, Pikachu calmed it down, and went straight to sleep. During a battle between the two trainers, Nebby woke up by all the commotion and let out a high-pitched scream, which knocked Pikachu back and got Lycanroc's fur clean. Pikachu praised Nebby for calming Lycanroc down, and when Ash realized how long he's been gone, he asked Nebby to teleport them back home, but it went back to sleep.
Nebby gets examined by Professor Burnet who gives it a clean bill of health and has grown to like her. Then, at the sleepover Nebby had fun playing a game of hide and seek with Ash and his friends and after the sleepover hearing Lillie's desire to see her brother Gladion. It teleports her to him then back to her friends, but Nebby's actions cause Lillie to become fearful of Pokémon again.
In an effort for Lillie to get over her fear, Nebby teleports her, Ash, Pikachu, and Snowy to various places from Lillie's memories as a child. Once they all made it to lab at the Aether Foundation, Faba tried to kidnap Lillie in an attempt to erase her memory. Nebby tried to teleport them away from danger but they were restrained Faba's Hypno; when Gladion's Sivally saved them and Lillie retained her full memory, she finally overcome her fear of touching Pokémon and Nebby was very happy for her.
Later, Faba successfully captured Nebby by tempting and feeding it candies and took it to his lab for experiments. After Ash and gang successfully stopped Faba and freed Nebby, it evolved into a Cosmoem.
After what happened at Aether Foundation Ash becomes worried about Nebby since it hasn't moved. When Ash and his friends discover that Lillie is nowhere to be found they talk with Kahuna Hala and from him they all head over Poni Island and then the Altar of the Sunne in the hopes of finding Solgaleo, but they find all Tapu Guardians there instead.
As Ash and the gang arrived at the Altar of the Sunne on Poni Island, Ash told the Tapu Guardians about Nebby's motionless form. Tapu Fini and Tapu Lele performed a ritual around Nebby. And after a few minutes, the Altar of the Sunne activated itself and Nebby evolved into Solgaleo. In the process, Nebby was able recognize Ash and Lillie and accepted their request in helping to rescue Lusamine from the Ultra Beasts. After the Tapu Guardians gave Ash the Solganium Z and upgraded his Z-Ring into a Z-Power Ring, Nebby opened up the Ultra Wormhole and entered Ultra Space with Ash and the gang on its back.
Nebby and the gang made to Ultra Deep Sea where Nihilego is residing. At that same moment, they've found Lusamine and to their surprise, she and Nihilego were fused into a monstrous being. Lusamine tells them to back off and form a stone wall to prevent the gang from getting closer. As the gang tried desperately to break the wall, Nebby stepped forward, changed into its Radiant form, and shatters the wall. As they pursue Lusamine, eash member of the group were forced into fighting her Pokémon, who were possessed by Nihilego's power. Nebby remained with Ash and continued to chase after Lusamine.
As Ash and Nebby got closer to Lusamine, she flooded the area with poisonous liquid. But luckily, Nebby protected Ash from the poison due to it being part Steel-type, dodging Lusamine's attacks in the process. Afterwards, Nebby went back to aid the others with the help from Ash's Lycanroc. Ash and Pikachu succeeded in freeing Lusamine from Nihilego's control and Nebby carried everyone back to the Pokémon world.
Nebby departed the gang without saying good-bye to them. During Professor Kukui and Professor Burnett's wedding, Nebby was seeing walking on the sky and Ash and his friends wave goodbye to it. Nebby looks down on them, smiling and departs to places unknown.
Nebby, along with Lunala, was later attacked by Necrozma in Ultra Space and they both tried to escape through the Ultra Wormhole. Nebby tried to protect Lunala, but it ended up getting hurt by Necrozma's attacks.
Nebby reappeared, where it reunited with Ash and briefly fought against Necrozma after it absorbing Lunala. It was able to free Lunala from Necrozma, but then was injured by the electric net of Team Rocket. But it was freed by the Ultra Guardians and Nebby manages to damage the ship before Team Rocket flew away. Later, Necrozma recovered after separating from Lunala and goes against Nebby again. Nebby ends up getting absorbed by Necrozma and the Prism Pokémon flees through the Ultra Wormhole, now in its Dusk Mane form.
Nebby remains fused with Necrozma until Ash, Gladion, Kiawe and Lana transfer the power of their Z-Moves to Necrozma in the hope that it would depart from Nebby, which didn't work since Necrozma doesn't want to let Nebby go. After the people and the Pokémon of Alola give their Ultra Aura to Necrozma, Nebby is freed from it and after recovering, uses its signature Z-Move, Searing Sunraze Smash with Ash's help on Necrozma so it will provide enough energy to return to its original form. After Necrozma transform into Ultra Necrozma and restores Poipole's homeworld, Nebby returns to Alola along with the Ultra Guardians and Lunala, however, it is unknown where it went after they returned.
|Teleport||Now You See Them, Now You Don't!|
|Supersonic -||A Masked Warning!|
|Sunsteel Strike||Full Moon and Many Arms!|
|Flash Cannon||Full Moon and Many Arms!|
|+ indicates this Pokémon used this move recently.*|
- indicates this Pokémon normally can't use this move.
|Searing Sunraze Smash||Revealing the Stuff of Legend!|
|+ indicates this Pokémon used this move recently.*|
- indicates this Pokémon normally can't use this move.
- Haven Paschall (English as Cosmog)
- Nebby shares some similarities with Squishy:
- At their first appearance, neither one of them were recognized by the Pokédex in the anime.
- Both of them are Legendary Pokémon being cared for by a main character; Bonnie took care of Squishy and Ash is taking caring of Nebby respectively.
- Both of them were used to fulfill the plans of an evil organization.
- Both were given nicknames because they weren't recognized by Pokédex.
- Like Squishy and Meloetta, Nebby is a Legendary Pokémon traveling alongside the group.
- The way Ash holds Nebby (as Cosmog) is similar to how Misty held her Togepi and Azurill, Brock with his Happiny and Dawn with her Piplup.
- Nebby (as Cosmoem) shares some similarities as Ash’s two other Generation I Pokémon’s mid-evolve forms, Butterfree (as Metapod) and Charizard (as Charmeleon). Both of these three Pokémon are mainly have less episode appearances of their mid-evolve forms:
- Like Metapod, Nebby’s mid-evolved form is also at an immovable state.
- Like Charmeleon, Nebby’s mid-evolved form only appeared in 3 episodes, where they both evolved in their last third episode appearance.
- Nebby’s parents, the elder Solgaleo and Lunala entrusted their child to Ash is similar how Suzie entrusted Brock her Vulpix, until both Pokémon left the main casts' crews.
- Team Rocket thought Nebby was a premilinary form of Koffing, Proto-Koffing, before they evolved into a Solgaleo
- ^ SM044: A Dream Encounter!
- ^ SM045: Now You See Them, Now You Don't!
- ^ SM046: Deceiving Appearances!
- ^ SM047: A Masked Warning!
- ^ SM048: Night of a Thousand Poses!
- ^ SM049: Mission: Total Recall!
- ^ SM050: Faba's Revenge!
- ^ SM051: Family Determination!
- ^ SM052: Revealing the Stuff of Legend!
- ^ SM053: Rescuing the Unwilling!
- ^ SM054: 10,000,000 Reasons to Fight!
- ^ SM055: The Professors' New Adventure!
- ^ SM087: Filling the Light with Darkness!
- ^ SM088: Full Moon and Many Arms!
- ^ SM089: The Prism Between Light and Darkness!
- ^ SM090: Securing the Future!