Pikachu is a short, chubby rodent Pokémon. It is covered in yellow fur, and its ears are long and pointed with black tips. It has a small mouth, brown eyes, and two red circles on its cheeks. There are pouches inside its cheeks where it stores electricity.
Its forearms are short, with five fingers on each paw, and its feet have three toes.
It has two brown stripes on its back, and its tail is in the shape of a lightning bolt with a patch of brown fur at the base.
A female will have a V-shaped notch at the end of its tail, as shown below:
It is classified as a quadruped, but it has been known to stand and walk on its hind legs.
Pikachu has been known to build up energy in its glands, and will need to discharge to avoid complications. It is also able to release energy through its tail, which acts as a grounding rod.
Pikachu first appears in Pokémon Red and Blue where it can be found as a wild Pokémon.
Due to the anime series (See the 'In Anime' entry), Pikachu is very popular and the Pikachu version was made where it is the starter Pokémon instead of the usual 3 starters; Much like Ash's Pikachu in the anime, this Pikachu will refuses the thunder stone and doesn't like to stay in the Poké-ball. (unless it is traded)
In the original games, Pikachu's design is much bulkier. Its design was changed to be more slimmer, to reflects the design changes in the anime series.
In almost every generations of the game series, there's a least one 'Pikachu clone'; Pichu (Pikachu's pre-evolve form) in Generation II, Plusle and Minun in Generation III, Pachirisu in Generation IV, Emolga in Generation V and finally, Dedenne in Generation VI.
They are all electric type Pokémon as Pikachu was.
In the main game series, while Pikachu has known to have 'Quick Attack' move that has higher attack power, Pikachu's base stats is low, especially the base defense stat.
However, Pikachu can held the item 'Light Ball'; It will doubles Pikachu's base attack and special attack stats. Futhermore, if one of the parents of Pichu holds a Light Ball, the Pichu that hatches will have the move 'Volt Tackle'.
The Light Ball is unobtainable in the games other than generation III, Generation IV and generation VI games, where the Light Ball only comes with the wild Pikachu.
In Generation VI games, Pikachu's digitally-composed voice (that is uses since Generation I) was replaced by Ikue Ohtani's voice. This does not apply to Pichu and Raichu.
Pikachu has been in every installments as a playable character since the very first Super Smash Bros. game.
In the main anime series, Ash received a male Pikachu from Professor Oak as he was very late to arrive in Oak's laboratory and all of the prepared starters are taken by other trainer. When Pikachu first meet Ash, he doesn't like him, doesn't take order from him and even electrocutes him. He does not like being in its Poké Ball.
Since episode 2, Pikachu had opened up to his trainer completely and was performing his own selfless acts.
As of current anime runs, Pikachu is extremely loyal to Ash, and while he is not always immediately trusting of strangers, he is generally friendly to humans and Pokémon alike.
Ash's Pikachu is Ash's most trustworthy Pokémon and is a main character in anime series that appears in every episodes of the every seasons.
Other Notable Pikachu in the main anime series
Sparky (Leon in Japanese)
Sparky first appears in episode 78 of the original series, under the ownership of Ritchie. It has the a scruff of fur on the top of its head.
Sparky, along with its trainer Ritchie, appear again in the end of episode 219, and episode 221 to 222 of the original series's Johto arc.
Sparky appear in flashback in episode 116 of the Best Wishes series.
Puka (Michael in Japanese)
Puka is a male surfing Pikachu who only appear in episode 67 of the original series. He has the blue eyes as opposed to the black eyes as normal Pikachu has, and he has wrinkles beneath his eyes and bigger and older than Ash's Pikachu.
This Pikachu is known to surfing in said episode.
Due to its popularity thanks to the anime series, there are a lots of Pikachu-related illustrations in pixiv.
There are also some Actor Joke illustrations involving Pikachu with other Ikue Ohtani's roles in any ways, such as Tony Tony Chopper from One Piece and Candy from Smile PreCure!, as shown below:
Ikue Ohtani (Ash's Pikachu in the main anime series, Pikachu in Generation VI games and Super Smash Bros. series)
Satomi Kourogi (Sparky in the main anime series)
Chika Sakamoto (Puka in the main anime series)