noun
A person who takes advantage of situations for personal gain, often without principles.
彼は便乗主義者で、いつも他人の成功に乗っかろうとする。
He is an opportunist, always trying to ride on others' success.
便乗主義者は状況が変わるとすぐに意見を変える。
Opportunists quickly change their opinions when the situation changes.
日和見主義者 emphasizes waiting to see which way the wind blows before acting, while 便乗主義者 focuses on actively exploiting an existing opportunity.
Compound of 便乗 (opportunistic riding along) + 主義 (principle, -ism) + 者 (person).