Pikachu

Pikachu is a short, chubby rodent Pokémon. It is covered in yellow fur with two horizontal brown stripes on its back. It has a small mouth, long, pointed ears with black tips, and brown eyes. Each cheek is a red circle that contains a pouch for electricity storage. It has short forearms with five fingers on each paw, and its feet each have three toes. At the base of its lightning bolt-shaped tail is a patch of brown fur. A female Pikachu has a "V"-shaped notch at the end of its tail, which looks like the top of a heart. It is classified as a quadruped, but it has been known to stand and walk on its hind legs; therefore, Pikachu is a facultative biped.

Pikachu stores electric energy using the small pouches in its cheeks in case it is attacked, so one might feel a small shock if it is touched. These pouches become electrically charged during the night as it sleeps, so a dozy Pikachu may occasionally discharge electricity after waking up. If it does not get enough sleep, it will not be able to release said electricity at its full power. When angered or threatened, Pikachu quickly unleashes the electric energy from its cheeks, which can reach the intensity of a lightning bolt. This also serves as a sign that it's wary, and it will feel stressed if it's unable to fully release the electricity. In Sparks Fly for Magnemite (EP030), Pikachu is shown to accumulate energy in its glands, which it will need to eject to avoid complications. When several Pikachu form a group, they are capable of causing thunder storms by building up electricity. If one Pikachu sees another one who is weakened and in need of help, it revitalizes it by recharging it with an electric shock. In Alola, a plan was recently announced to help build an electric power plant by gathering many Pikachu. If its cheek sacs are very soft and stretchy, this means that it can generate powerful electricity. As shown in Pokémon Sleep, Pikachu is known to emit electricity through its cheeks while sleeping, perhaps because it's dreaming of firing electric shots.

Pikachu is a curious, playful, and intelligent Pokémon who primarily eats fruits and roasts Berries with its electricity to make them tender enough to eat. However, it can end up accidentally burning them too much if it underestimates the intensity of its charge, as it has a tendency to blast new things it finds. In The Electric Tale of Pikachu, it is also depicted eating and sometimes destroying telephone poles, wires, and other electronic equipment. Pikachu raises its tail to monitor its surroundings and bites people who yank it, as it does not like having it pulled. While doing this, its tail is sometimes struck by lightning. Wild Pikachu live in forested areas away from people and with others of its kind in nests, as seen in the Pokémon the Series episode Pikachu's Goodbye (EP039). Such forests are dangerous, though, as trees there are often struck by lightning. When Pikachu meet, their tails make contact and they exchange electricity through them to greet each other. Relatedly, in A Plethora of Pikachu! (SM091), it is shown that Pikachu from Alola are known to greet each other by sniffing one another and rubbing their tails together. In the Pokémon Adventures manga, it is considered exceptionally rare, though it seems to be a quite common Pokémon in other forms of media. Many Pokémon Trainers prefer Pikachu's cute appearance over Raichu's, which is why the latter is not commonly seen among them. Because of the popularity of Pikachu-themed merchandise, Mimikyu dresses itself in a Pikachu cloth in an effort to look less scary and make friends and appease its loneliness. Unfortunately, its creepy costume only ends up doing the opposite. By occasion, Pikachu can get unintentionally swallowed by Cramorant. While startled, it looks for ways to escape from the latter. Pikachu, along with Pichu and Raichu, is related to Pachirisu.

Forms
Pikachu has sixteen additional forms that fall into four groups: Cosplay Pikachu, Pikachu in a cap, the partner Pikachu, and Gigantamax Pikachu. Additionally, many other Pikachu variants have appeared in various media.

Cosplay Pikachu
Cosplay Pikachu is always female, has a black heart-shaped spot at the end of its tail, and can be dressed in any of five outfits corresponding to the five Contest conditions.

Pikachu in a cap
Pikachu in a cap is always male and has eight variants, each wearing one of Ash's hats from Pokémon the Series. Pikachu in a cap is the only known Pokémon capable of using the Z-Move 10,000,000 Volt Thunderbolt. Pikachu in a Cap cannot use Catastropika or Gigantamax.

Partner Pikachu
The partner Pikachu is the Pokémon the player starts with in Pokémon: Let's Go, Pikachu!. This Pikachu can be either gender. It has higher base stats than normal Pikachu. Unlike other Pikachu, the partner Pikachu can learn the exclusive moves Zippy Zap, Floaty Fall, and Splishy Splash, as well as having access to the Partner Power Pika Papow.

The partner Pikachu is not differentiated from standard Pikachu in Pokémon: Let's Go, Pikachu! and Let's Go, Eevee!'s Pokédex, and it cannot be transferred out of the games.

Gigantamax Pikachu
As Gigantamax Pikachu, it becomes larger and chubbier. Its tail becomes longer and becomes capable of storing all of the electricity it generates, causing it to glow a bright yellow and enabling it to become as powerful as a lightning strike. Three red clouds float around the tail. Its power is equal to that of a power plant; however, it is difficult to use it in peoples' homes since Pikachu can only remain in its Gigantamax form for a short time.

Mr. Windychu & Ms. Wavychu
Mr. Windychu and Mrs. Wavychu will be characters in Pokémon Winds and Waves respectively.

They resemble a tanned Pikachu with clothes similar to the playable character. No further information is known.

Evolution
Pikachu evolves from Pichu and evolves into Raichu.
In Alola, it evolves into Alolan Raichu.
Cosplay Pikachu, Pikachu in a cap, Partner Pikachu, and Pikachu with the Gigantamax Factor cannot evolve.

(Source: Bulbapedia)