During each Mission Cycle there is only one Guardian Squad, a small group of people with one core Mission, one sacred duty:
"Protect the Hero Squads from losing focus on succeeding their missions."
Being a Guardian should be a rotating role, giving equal opportunities to everyone to join Hero Missions. Guardian Squad is actually a support team, working on incoming bugs, support engineering or other ad-hoc requests that usually span to no more than a few days.
Guardian Squad is the only constant force in Agile Squads and usually all work items of this team, could be summarized in a Guardian Backlog, that is constantly consumed from cycle to cycle, even if the human ingredients of this team changes.