If i remember correctly, mc is mc, "senior" is actually an assassin hired by some oligarch, redhead reporter is kinda insane and really wants the scoop, her assistant kinda looks up to insane reporter, the fake vigilante is a rich bastard son of a CEO and he abs loves the mc, mc's station chief currently suspects him, mc's girlfriends are wholly irrelevant since they are just alibis.
Cant quite remember who the recently dead reporter was but i think he was investigating the oligarch who hired the senior cop, but was likely offed by another dude the oligarch hired
Corrections:
Senior is really a cop, but is heavily intimated to be a black-ops type with a past of shady jobs. He was put in charge of the investigation by the corrupt chief commissioner of the Korean police, who's actually in cahoots with the oligarch the reporter and her assistant are investigating. The chief commissioner tasked him with offing the Vigilante while finding some loser to take the blame, lest the public become incensed by the finding that the Vigilante is in fact an upright man.
The Imilante is not the same oligarch as the one who ordered the killing of the reporter: the latter is a violent guy who's under the Imilante (not knowing his alter ego, of course).
The one who suspects Ji Young of being the Vigilante is not the station chief, but one of his subordinates. And there's a professor in the National Police Academy who also suspects Ji Young.
There are no "girlfriends", this girl is indeed an alibi and this is their first time meeting.
As for insanity… you're right, everyone in this series except for the reporter's assistant is insane. Reporter Chae probably takes the crown, she's batshit crazy.