Grojband follows Corey and his three best friends, Laney and twin brothers Kin and Kon, as they work to propel their garage band to international stardom. When they don't have the lyrics, Corey and his friends get Trina into an emotional diary mode to write lyrics in her diary, so that Corey and his friends can perform a perfect song.
Corey Riffin (voiced by Lyon Smith) – Corey is the charming and quirky 13-year-old leader, vocalist, main protagonist of the series and guitarist of Grojband. Although kind, he will do whatever it takes to play music, which is usually by angering his sister, Trina, who will write in her diary. He then translates her venting into lyrics for their songs.
Laney Penn (voiced by Bryn McAuley) – Laney is the bassist and self-proclaimed band manager of Grojband. A fiery redhead, she is the only female in the band. She tends to be more rational than the rest of her group, but is prone to giving in to Corey and his schemes, mainly due to her crush on Corey. A running gag on the show is despite her clearly being female, nearly everyone assumes her as a boy.
Kin Kujira (voiced by Sergio Di Zio) – Kin is the keyboardist of the band, and Kon's twin brother. He has a knack for technology, and usually builds strange gadgets that help the band.
Kon Kujira (voiced by Tim Beresford) – Kon is the drummer of the band, and Kin's twin brother. Kon is overweight and sometimes dense, but is usually well-meaning with a childish demeanor.
The Newmans are a gender-reversed doppelgänger band to Grojband.
Carrie Beff – Corey's doppelgänger: the leader, guitarist, secondary antagonist and vocalist of The Newmans. She is also Mina's sister.
Larry Nepp – Laney's gender-bend rival: bassist and band manager of The Newmans.
Kim Kagami – Kin's gender-bend rival, who is also the keyboardist of The Newmans.
Konnie Kagami– Kon's gender-bend rival, who is also the drummer of The Newmans.
Trina Riffin (voiced by Alyson Court) – Trina is Corey's older sister of 16 years of age, and the main antagonist of the series. She is always determined to ruin Corey's life and be the best around the house. She is spoiled, rotten, and has a huge crush on Nick Mallory, despite him barely acknowledging her existence. Whenever she gets into a fit of rage, she impulsively writes her diary, which provides Corey with lyrical inspiration for their songs.
Mina Beff (voiced by Denise Oliver) – Mina is Trina's best friend and lackey. Despite wanting to express her own opinion, she wishes to be as cool as Trina, and does whatever she tells her to do.
Nick Mallory (voiced by Graeme Cornies) – Nick is the coolest kid in school, whom all the girls in school adore, especially Trina, even though he is oblivious to her existence. He is quite narcissistic, and refers to himself in the third person, but isn't inherently antagonistic, and will sometimes aid the band when they see use of him.