Pikachu is a short, chubby, rodent-like Pokémon with yellow fur all over its body. Pikachu's ears are long and come to a point with black tips. It has a small mouth, which resembles a sideways 3, it has black eyes and two red circles on its cheeks. Its forearms are short and somewhat stubby, with 5 digits on each "hand" and its feet have three digits. It has two brown stripes on its back, and its tail is in the shape of Zigzag-styled lightning bolt, with a patch of brown fur at the base of the tail. Although it is technically a quadruped, on multiple occasions it has been shown to be able to stand and walk upright on its hind legs.
Besides its evolution and pre-evolution, it may be related to Plusle, Minun, Pachirisu and/or Emolga due to their many physical similarities.
A female Pikachu has a v-shaped "dent" at the end of its tail. Male Pikachu do not have this dent. However, both genders' tails resemble a lightning bolt, fitting their Electric typing. In Black and White, male Pikachu have less black covering their ear, though this is only shown in frontal sprites.
Exclusive to Pikachu is the ability to release electric discharges of varying intensity, at will, through electric sacs in its cheeks. This electric energy can be manipulated into a variety of techniques which are mostly used in battle, but which can be used to roast Berries as well.
As Pikachu builds up energy in its electricity glands, it often needs to discharge, as to not result in a short, or worse, which can lead to physical paralysis, or in severe cases, death. Pikachu release the built-up energy through their tail, which acts as a grounding rod, to remove unneeded energy safely. A sure sign that a location is inhabited by Pikachu is to look for patches of burnt grass, which is the result of the discharge. Through an inability to release a buildup of electricity, a Pikachu may develop a rare condition similar to the human flu. This illness is most often caused by strong nearby electromagnetic forces, which severely affect the electric glands. If a Pikachu is introduced to a magnet, it will be attracted towards its cheeks and display other common attributes of magnetism.
In the wild, Pikachu often travel in packs, and are rarely territorial. However, when threatened, a group can generate an intense electrical offensive, and the electromagnetic forces exerted by the resulting field can even produce a short lived, localized thunderstorm. Pikachu can be harmful pests, eating and sometimes destroying telephone poles, wires, and other electronic equipment.
Despite this, it is a popular pet, and relatively easy to maintain, with little exercise necessary for its well-being. However, it is important to discharge the electricity in its cheeks periodically.
As many know, pulling on a Pikachu's tail will result in a severe shock to the offender. Another defense of Pikachu's is the use of its nasty bite, which has been known to leave quite a mark.
They are found mostly in forests, but possibly because of their attraction to electricity or simply because they are not afraid of people, Pikachu commonly show up in civilized areas. They may even enter houses, presumably attracted by the electrical output generated by the dwellers' appliances. Pikachu are native only to the Kanto region (specifically, Viridian Forest and the Kanto Power Plant), although they have been introduced to the Hoenn Safari Zone and to the Trophy Garden in Sinnoh.
Living in forested areas, Pikachu often are found foraging for Berries. Instead of climbing trees, they use small electrical shocks to release the Berries from the tree, roasting them at the same time.
As pets, they can be fed a variety of processed food. Alternatively, like most Pokémon, Pikachu are tolerant to most "human" foods, and sometimes enjoy them more than the more nutritional food designed for their biology. An example of this is the fact that Ash's Pikachu has been known to have a fondness of ketchup. Pikachu is also occasionally seen eating apples.
Ash was given the opportunity to release Pikachu in Pikachu's Goodbye, as Pikachu seemed very happy with the other Pikachu. Even though he did attempt to, the bond between the two was too strong, so Pikachu chose to stay with him. This episode also featured many other wild Pikachu.
Another Pikachu nicknamed Puka was owned by a man named Victor on Seafoam Island in The Pi-Kahuna. This blue-eyed Pikachu saved Ash from drowning by having the ability to sense tidal waves approaching.
In Mewtwo Strikes Back, Mewtwo clones Ash's Pikachu. This Pikachu can be told apart from Ash's due to spikes on the black marks at the tips of its ears. It is also comparatively more aggressive than Ash's Pikachu and slurs its speech, defects possibly caused by Ash tampering with Mewtwo's cloning machine during the cloning process.
James used a Pikachu in the Pokémon League Entrance Exam in The Ultimate Test, but it was defeated by the instructor's Graveler. James, after interfering with a battle Ash was having with the instructor, later tried to steal this Pikachu, only for that Pikachu to zap James instead, as instructed by its instructor.
A Pikachu named Sugar belonging to the owners of a restaurant went missing prior to Cooking up a Sweet Story!, and Ash's Pikachu had to fill in for it so that its owner would have the confidence to win a cooking competition. At the end of the episode, Sugar returns, however, it has evolved into a Raichu.
A Pikachu that belonged to an unnamed Aura Guardian appeared in a flashback in The Keystone Pops!.