Who are Satoshi and Pikachuu?

Satoshi

Game appearance: Pokémon_Puzzle_League
Anime appearance: Satoshi is one of the main protagonists of the series
Manga appearance: The Electric Tale of Pikachu, Pocket Monsters Zensho, Ash & Pikachu

Satoshi, the Pokémon trainer from Pallet Town, is named after Satoshi Tajiri, the creator of the Pokémon games. Satoshi (also known as Ash Ketchum in English) aspires to be a Pokémon Master, and does this by traveling through regions battling Gym Leaders and entering in Pokémon League competitions, similar to the protagonists of the Pokémon video games. He is first accompanied by Gym Leaders Misty and Brock, and later on by Tracey Sketchit in Pocket Monsters, and May and Max in Advanced Generation.

At the start of the series Satoshi's personality is largely that of the fairly stereotypical shounen hero: very excitable, empathic, sometimes focused to the point of distraction, and clever in regards to his interests but often little else.

His lifelong rival is Gary Oak. When he became 10 years old, he was to receive his first Pokémon. He overslept and by the time he arrived at Professor Oak's Laboratory, all the starter Pokémon (Bulbasaur, Charmander, and Squirtle) had been taken and he had to start with a Pikachuu instead. His Pikachuu has the particular characteristic of hating Pokéballs so it always tags along with Satoshi. Because of Pikachuu's powerful electric attacks, Satoshi is the frequent target of Team Rocket's plans of stealing Pokémon for their own interests.

The given names Ash and Satoshi were one of the default choices for names in Pokémon Red (and similarly, Gary and Shigeru for Pokémon Blue). The surname Ketchum is believed to be a pun on the phrase "catch'em". It is possible that he was given this surname to match the number of mouthflaps in the anime (Satoshi has 2 more syllabes then Ash)

In the English anime, Delia Ketchum, Satoshi's mother, oftentimes pesters him to change his underwear every day; the gag runs throughout the Kanto episodes. In the original Japanese, his mother's name is Hanako. The identity of Satoshi's father is unknown, as his parents are separated. For some time, it had been widely speculated that Satoshi's father may in fact be Giovanni, the head of Team Rocket. However, these are merely speculations based on the non-canonical musical Pokémon Live, in which it was revealed that Delia had a relationship with Giovanni while they were younger.

Pikachuu

Pikachuu is an electric mouse Pokémon, and the most popular and notable Pokémon. It is obtainable in all eight of the Pokémon RPGs and is the species of one of the main characters in the Pokémon anime, the Pokémon manga series (Electric Tale of Pikachuu), and the Ash & Pikachu manga.

Pikachuu is yellow with brown stripes on its back, black-tipped ears, red cheeks, and lightning bolt-shaped tails. It lacks a typical 'natural' evolution but can be evolved via Thunderstone into Raichu. However, Pikachuu is so cute that most trainers choose to keep it from evolving. It is numbered as #25 in the National Pokédex. Also, in the second and third generations of the Pokémon games, Pikachuu has an evolutionary predecessor, Pichu. Pichu will evolve into Pikachuu through happiness.

The name "Pikachuu" is probably a portmanteau, derived from "pika", meaning "spark" or "sparkle" of lightning in the Japanese language, and "chu", an onomatopoeia in the Japanese language for the squeak made by a mouse.

Above information gathered from several sources, but mainly Wikipedia