altruist
A person who practices altruism; a somewhat formal or philosophical term.
He is a true altruist.
利他主義 is the abstract concept of altruism, while 利他主義者 refers to a person who embodies it.
Compound of 利他 (rita, 'altruism, benefiting others') + 主義 (shugi, 'principle, -ism') + 者 (sha, 'person').