Here is the full list of all 26 Feign achievements.
Win 5 games with imposters.
Win a game with Cleaner
Win a game with Haunter
Win a game with Blocker
Win a game with Stealer
Win 5 game with Neutrals
Win a game with Sorcerer
Win a game with Serial Killer
Win a game with Tracker
Win a game with Snitch.
Win a game with Bomber
Win a game with Provoker
Win a game with Blamer
Win 5 game with Innocents.
Win a game with Insane
Win a game with Trapper
Win a game with Lookout
Win a game with Doctor
Win a game with Survivor
Win a game with Investigator