She is best known for being the lead singer of Dutch symphonic metal band Epica, which she joined at the age of seventeen, releasing eight studio albums and touring the world.

[5] At age 15 she switched from pop/jazz singing lessons to study as a lyrical singer, becoming also interested in classical music and composition, after listening to Nightwish's second album Oceanborn and hearing Tarja Turunen's voice for the first time.

[9] In late 2002, the band courted Helena Iren Michaelsen[9] (from Trail of Tears) as its frontwoman, but shortly after she was replaced by the then unknown Simons, who was Jansen's girlfriend at the time.

[10] Asked about her looks as one of the factors that attract attention to Epica, Simons said: "First of all the most important is the sound of the band, because beauty will fade one day, and I hope my voice won't [Laughs]...

[15] Simons runs a blog called "SmoonStyle", where she writes about her fashion, makeup, food and experiences she has had as a member of Epica, as well as in her personal life.