Everyone loves Pikachu, whether you’re watching the show or a character in it. The electric mouse Pokémon that stars alongside his trainer in the anime has become an icon in worldly culture. Pikachu has definitely made us laugh and even made us cry; the huge amount of screen time he gets gives him a huge advantage as a character, showing a more diverse personality, more development and more characteristics than any other Pokémon in the show.
When Ash first received Pikachu, he wouldn't do anything he was told to do, nor did he want to go back to his Poké Ball. When shown affection, Pikachu would return a shock, when told to do anything he would resist, not because he didn't want to or it would have caused him any discomfort, but he seemingly wanted to resist orders and be awkward for Ash, and some people remember Charizard as the first to disobey Ash. Pikachu showed pleasure at Ash's attempts to catch the weakest of Pokémon, which may suggest Pikachu knew enough about Pidgey and Rattata to recognize them as generally weak Pokémon. Of course, he may have just taken pleasure in watching Ash's misfortune but his tune changed when it came to Spearow. Pikachu appeared terribly afraid when he was attacked by Spearow and was very quick to flee, of course not caring what happened to Ash. It took a great act of bravery and selflessness to win Pikachu over suggesting he suffered some betrayals or let downs in the past leading him to be so closed-minded and unwelcoming of others. By the end of the second episode he had opened up to his trainer completely and was performing his own selfless acts. Pikachu's electricity combined with the explosive make-up of Koffing led him to be pursued by Jessie, James and Meowth for the next 13 seasons, characters he'd later form much greater relationships with.
Pikachu did stay quite lazy while he was inexperienced as a battler, afraid to take on powerful opponents like Pinsir and Onix but had no problem when around smaller friendlier looking Pokémon like Caterpie and Clefairy, returning the friendliness. On many occasions Professor Oak has said Pokémon take on the traits and feelings of their trainers, in Pikachu's case, while Ash trained him, Ash, Misty and Brock all cared for him and rubbed off their personalities on him.
Like Ash and Brock, Pikachu soon ceased caring if he made a fool of himself and soon welcomed dressing up and acting silly for fun and like Misty, stopped at nothing to make sure he was heard, the best example would be when the two courageously stood up to the giant Tentacruel terrorizing Porta Vista.
When Pikachu met a smaller, younger Pikachu and more so when Togepi was born, he displayed responsible, caring, big brotherly traits and took it upon himself to look after them. This gives me the impression Pikachu is quite young as it is a vital stage in a child's social development when they become keen to show off compassion and responsibility for those younger than themselves. Being with Togepi for so long seemed to imprint the trait into Pikachu's character as he would later act the same way towards Phanpy, Torchic, Pachirisu, Happiny, Axew and even Max.
Let's play Mummies and Daddies
The weight of constantly caring for another Pokémon was lifted in the Diamond & Pearl series when Pikachu was given a best mate, besides the obvious human Ash. As well as his best friend Piplup, he also gained a rival in Paul's Elekid, after always having distinct relationships with members of the Electabuzz evolution family, whenever they appeared. Pikachu's social placement developed further when he caught the eye of a Buneary that joined Dawn's team. When faced with a possible romance Pikachu usually seemed oblivious, much like Ash is with the attention he receives from the ladies. However, it was not always shown to be one-sided as Pikachu has comforted Buneary similarly to how he would the younger Pokémon and been bashful around her. Whether Pikachu's feelings for Buneary are neutral is uncertain but it is obvious he welcomes the friendship. Within his own subculture Pikachu picked up traits from his friends, most notably Piplup, another reason to suggest he is quite young. More discreetly than Piplup, Pikachu began to overreact slightly over small problems and showed even more comic traits. In the last episode of the DP series Pikachu burst into tears seeing Piplup doing the same over the gang parting ways. It is obvious Pikachu developed a strong relationship with Dawn and every other friend that traveled with Ash and him for a long while before leaving them.
Working with Team Rocket?
Well, I guess I don't want to drag around your corpse
Typically, Pikachu as the main protagonist is not fond of the antagonists of the show, Team Rocket. Usually he'll waste no time in getting them out of his sight and at times he appears to enjoy sending them blasting off, this is probably his original self shining through, but like with Ash it soon deteriorated. Pikachu, despite being a victim of Team Rocket's constant harassment will risk his own well-being for the trio and sensibly recognizes when he has to turn the other cheek and work with them to overcome other problems. Many times he has become lost with Meowth; Pikachu keeps Meowth safe in these endeavors as he feels it is morally right. However, Pikachu shows absolutely no respect for Meowth and even when working together keeps an eye on him. On the occasions where Meowth tries to take the lead, Pikachu will arrogantly ignore him as if Meowth was completely stupid.
As well as all this, Pikachu is one of a few Pokémon in the show that does not want to ever evolve, a similarity he has with Best friend Piplup and love-hate 'frienemy' Meowth. From a psychoanalyst's point of view this may be because of an experience when he evolved from Pichu. (Though in reality Pikachu's resistance of evolution was introduced before Pichu was.)
Pikachu brought out a lot of personality from other Pokémon in the anime. I do not believe any of the main cast Pokémon are plot devices with no personality, even if they only showed it in the episodes they were caught, released or evolved in, and I shall be exploring them all as characters from here on in.