Okay to be clear. spoilers:end of spoilers
Jason is both, he is the detective with the white/light hair and he is also the costumed Joker vigilante that's working alongside Batman in the story. Detective by day, crimefighting vigilante by night.
Another source of confusion seems to be that spoilers:end of spoilers
since there's actually another more traditional evil 'Joker' like character featured in the series, people make the mistake of assuming that that must be Jason, (since they hear the rumours that Jason is the Joker) but I assure you it isn't.