noun, noun which may take the genitive case particle 'no'
anti-mainstream; fringe
Refers to ideas, movements, or people that oppose or exist outside the dominant trend or mainstream culture.
彼は反主流の意見をよく述べる。
He often expresses anti-mainstream opinions.
そのアーティストは反主流の活動で知られている。
That artist is known for fringe activities.
カウンターカルチャー is a loanword for 'counterculture', often used for broader cultural movements, while 反主流 can be used more generally for any anti-mainstream stance.
Compound of 反 (anti-) and 主流 (mainstream).