1) Active 2) Activistic 3) Advocate for action 4) Campaigner 5) Champion 6) Crusader 7) Demonstrator 8) Dissenter 9) Fighter 10) Issue-oriented person 11) Marcher 12) One with a cause 13) Picketer 14) Promoter 15) Protester 16) Rights fighter 17) Striker 18) Supporter 19) Vigorous cause advocate 
An activist is someone who supports or fights for a campaign or cause by standing up for it either by campaigning, protesting etc.
[adj] - advocating or engaged in activism
activistic adjective advocating or engaged in activism
an especially active, vigorous advocate of a cause, esp. a political cause.