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The central "secret" of the series is that Jem is in fact the alter ego of Jerrica Benton, owner/manager of Starlight Music, who adopts this persona with the help of Synergy, an extremely sophisticated holographic computer designed by her father and left to her after his death.
The other Holograms are Kimber Benton, Jerrica's younger sister, keyboardist and main songwriter for the band; Aja Leith, guitarist; and Shana Elmsford, who plays the synth drums. Aja and Shana are also childhood friends and adopted foster sisters of Jerrica and Kimber. Shana briefly leaves the group in a two-part episode, and a new character, Carmen "Raya" Alonso, is introduced as her replacement. When Shana returns, Raya remains the Holograms' drummer, while Shana takes up the guitar.
Only the Holograms are aware of Jem's secret identity (although Jem revealed it to the President of the United States in the episode "The Presidential Affair"), and a small Chinese girl named Lin who discovered the secret in "Adventures in China." Episodes of the series frequently revolve around Jerrica's efforts to keep the two identities separate. This often causes conflict with her boyfriend, Rio, who is attracted to both Jerrica and Jem while not realizing that they are actually the same person.
Jerrica's other main concern is Starlight House, a home for foster girls run by the Holograms. The original location was Jerrica's childhood home until it was burned by a henchman of Eric Raymond. Later Jem and the Holograms won possession of the New Starlight Mansion during a battle of the bands. The Holograms' main purpose is in fact to fund the Starlight Foundation and support the Starlight Girls. The mansion, as well as a lucrative recording contract was offered by movie producer Howard Sands, who becomes one of the group's biggest supporters. Other friends of the Holograms included Countess Danielle DuVoisin, a renowned fashion designer, Anthony Julian, a music producer who is romantically involved with Shana, charity worker and choreographer Danse (Giselle Dvorak), and Craig Phillips, who entered the contest for the Holograms new drummer and is the older brother of the Misfit Stormer. He has an ongoing relationship with Aja.
The Holograms' rival band are the Misfits: spoiled rich girl Pizzazz (Phyllis Gabor) and her cohorts: no-nonsense and streetwise Roxy (Roxanne Pelligrini) and kind-hearted and sensitive Stormer (Mary Phillips), who are later joined by British saxophone player and scam artist, Jetta (Sheila Burns). Most episodes of the series involve a plot by the Misfits to sabotage and/or upstage Jem and the Holograms' latest glamorous escapade. This rivalry is encouraged and manipulated by their manager, the ruthless Eric Raymond, who was the business partner of Jerrica's father. Eric ran Starlight Music after Jerrica's father's death until Eric had a falling out with Jerrica. Eric stakes his claim on the record company against Jerrica's claim, thus starting the rivalry between the Holograms and the Misfits. Although in the final episode of the series, both groups seemed to have declared a truce between them.
The final season of the show introduced a new set of antagonists, a new band called the Stingers, consisting of: Riot (Rory Llewelen), Rapture (Phoebe Ashe), and Minx (Ingrid Krueger). Riot also added romantic drama to the show, as he was obsessed with Jem. Pizzazz, in turn, was obsessed with Riot, and Rio was pursued by Minx. Dolls of the Stingers were planned, but not manufactured.
Other characters important enough to have related dolls were filmmaker Video (Vivian Montgomery), her cousin and obsessive Misfits fan Clash (Constance Montgomery), choreographer and Danse, and three of the most prominent Starlight Girls: Ashley, Ba Nee, and Krissy.
Special thanks to Wikipedia!