The noun 斗士 (dòushì) means 'fighter,' 'warrior,' or 'champion of a cause.' It is composed of 斗 (to fight) and 士 (person/scholar). It can refer to a literal combatant, as in 角斗士 (gladiator), or figuratively to someone who fights for a cause or belief.
他是一位为自由而战的斗士。
Tā shì yí wèi wèi zìyóu ér zhàn de dòushì.
He is a fighter who fights for freedom.
她一直是环保事业的斗士。
Tā yìzhí shì huánbǎo shìyè de dòushì.
She has always been a fighter for the cause of environmental protection.
那部电影讲的是古罗马角斗士的故事。
Nà bù diànyǐng jiǎng de shì gǔ Luómǎ jiǎodòushì de gùshi.
That movie is about the story of ancient Roman gladiators.