Pikachu is Shinsuke's very first Pokemon and he was with him since he was young. It is unknown if Pikachu was ever a Pichu. Many people made the joke, that Shinsuke resembles his Pikachu alot. Its currently known moves are Thunderbolt and Quick Attack.
Shinsuke found his Two Pichu alone in a Forest and decided to take them with him. One of the Pichu is Female, and the other Male. Unlike his Pikachu, the Pichu love staying on their Pokeballs. Their moves are currently unknown.