From Bulbapedia, the community-driven Pokémon encyclopedia.
Sandy (Japanese: ナギサ Nagisa) is a character who appears in the manga series Magical Pokémon Journey. She makes her first appearance in the chapter Raichu's Best Friend.
Sandy is the ghost of a girl who lived 150 years ago. In her former life, she owned a Raichu. Sandy's parents left Raichu in a box by the sea, because it had been accidentally shocking people with its electric power. Sandy promised to meet Raichu, but before they were able to meet, they were both killed by a tidal wave. Raichu, who remained as a ghost as well, was bitter because he thought that Sandy had abandoned him, however, with the help of Pikachu and his friends, his ghost is reunited with Sandy's and they depart for the afterlife.
Sandy and her Raichu appear again in Heavenly Pokémon, she helps Eevee and Lu when Eevee is possessed by the spirit of a lovesick Golduck.
In The Miraculous Color of the Sky ~Best Friends~, her and Raichu are reincarnated as a little girl and a Pichu. They meet again in their new lives and become friends again.