Marcus Holloway (born 11 December 1992), alias Retr0, is an American vigilante, hacktivist, and grey hat hacker that is a member of DedSec's San Francisco, California branch. At one point on the FBI's Most Wanted list, Holloway worked to destabilize Blume and sabotage ctOS 2.0 due to ctOS's invasiveness and Blume's perpetuation of corruption and greed.
In 2016, Holloway was responsible for several major arrests and deaths in San Francisco, including the arrests of congressman Mark Thruss and Blume CTO Dušan Nemec.