Diamandis was born in Abergavenny, Wales to a Greek father and a Welsh mother, and brought up in the village of Pandy with her parents and her older sister. She attended Haberdashers' Monmouth School for Girls, of which she said "I sort of found my talent there... I was the one who always skived off choir, but I had an incredible music teacher who managed to convince me I could do anything." When her parents separated, Diamandis moved to Greece when she was sixteen years old to live with her father but returned to Wales two years later.
Diamandis moved to London at the age of eighteen where she attended dance school for only two months. Following this, in 2005 she took a one year singing course at Tech Music Schools. Diamandis enrolled on a degree in music at the University of East London, transferring in her second year of studies to Middlesex University, but she later dropped out. She went for many auditions including the West End musical, The Lion King. Diamandis admitted that she auditioned for a reggae boy band, held by Virgin Records, in 2005 to try to make it into the music business. She said she was "delusional with drive" and ultimately decided to dress up in male attire to try to amuse the record label to sign her, but she was unsuccessful. However, she was called back by the record label a week later.
Diamandis has a synaesthetic condition that involves seeing musical notes and days of the week in different colours.[2