Jason Aalon Butler is an American vocalist and songwriter best known as the former frontman of post-hardcore band letlive. and the current lead vocalist of Fever 333, a band blending elements of rap, punk, hardcore, and activism-driven lyrics. Butler is recognized for his high-energy performances, raw vocal intensity, and outspoken approach to social and political issues in his music. His projects have been praised for pushing genre boundaries, combining aggression with hip-hop rhythms and melodic sensibilities.

Beyond his work with Fever 333, Butler has become a prominent figure in the modern alternative scene, collaborating with artists across rock, hip-hop, and experimental music.

His connection with ONE OK ROCK is highlighted through his contributions to their 2025 album DETOX, where he co-wrote and composed two tracks: Delusion:All and +Matter. These songs blend his raw, socially conscious style with ONE OK ROCK’s melodic intensity, resulting in some of the album’s most powerful and provocative moments. This collaboration reflects the band’s ongoing commitment to incorporating global influences and pushing creative boundaries.