A “minor antagonist” is an antagonist role which can exist in any roundtype, either by admin spawn or as a ghost role.
The main distinction between standard and minor antagonists is level of severity, usually a minor antagonist will be getting up to no good, but not seeking to murder large portions of the ship/station. Your exact concept will depend on the type you've rolled.
A thug is usually a disgruntled person or crewmember, cleared to break the law and crack skulls. Something or somebody really pissed them off, and now they're going to break something.
A renegade is the “good guy with a gun” archetype, for better or worse. Their primary goal is to defend themselves, usually using an unlicensed weapon they've smuggled aboard. If somebody crosses you, they're going to pay.
Note that a renegade is not an official member of security (and may not even be a crewperson) and will probably run into trouble with the law for their vigilante actions.
Ghost roles are selected from eligible observers in-round. This includes stowaways, cursed swords, and other weirdness. They should follow their roleplay guidelines found in the flavour text.
A stowaway is a ghost role spawned in mid-round. This can be 'just' a stowaway, or a combination with Thug or Renegade.
You will spawn in maintenance as a trespasser, and are not a member of the crew. How the crew reacts to this is likely to vary greatly, so be cautious.
A debtor is a ghost role spawned in mid-round. This can be 'just' a debtor, or a combination with Thug or Renegade.
You spawn hiding in maintenance, and are a known Debtor to the ship (a former passenger or crewmember that lost their right to be aboard but hasn't vacated yet.) How the crew reacts to this is likely to vary greatly, but expect disdain or worse. You do get the special power to determine what's in maintenance pills, so you could use that power to be helpful.