Refers to a person who actively fights for a cause, ideal, or in a literal combat role. Often used in political, social, or sports contexts. The gloss 'boxer' is a specific sports usage.
He is known as a fighter for freedom.
The politician calls himself a champion of reform.
He is known as a fighter for the human rights movement.
She fought as a champion for freedom.
He eventually became a fighter for the anti-war movement.
ファイター is the casual loanword for a fighter, especially in sports like boxing, while 闘士 is more literary and can apply to political or ideological struggles too.
闘士 is a person who fights for a cause (a fighter), while 闘志 is the abstract fighting spirit or will to fight, despite the identical reading.
Compound of 闘 (fight) and 士 (person of a certain status or profession). The exact historical derivation is uncertain, but the word has been used since at least the early 20th century for fighters in ideological or physical struggles.