Pikachu is Erbby's best friend. Pikachu saw something very special in Erbby who see him as a trainer with a good heart, which is why he follow Erbby while he went to begin his journey as Trainer and Pikachu became his Starter Pokemon. Pikachu seem to care a lot about Erbby as he only share it true feelings with him as he also love Pokemon and see them as friends and family and his Trainer view him as a brother. Pikachu always been on Erbby's side and will have his back the same way he does with him.
Like his Trainer, Pikachu also love to battle, play Video-Games and be on a whole lot of adventures. Pikachu also know some secret techniques which learn by some people, he also favors cop down and Sea Skim the most. Also Pikachu doesn't want to evolve because in want to be stronger as the way it is to prove that Pokemon strength is more than just evolution.
Pikachu is also friends with other people and Pokémon other than its Trainer, such as Elaine's Eevee.
Pikachu and his Trainer also admire Ash Ketchum and his Pikachu as they started the same Journey as they did. While his trainer worship Ash, Pikachu worships Ash Pikachu as he show it to be a great battler prove how strong it is without evolving. Both Pikachu and Erbby look to them as their heroes as they want to great like them one day.