The Minutemen
Led by Agent Graves, the Minutemen are seven killing machines who policed The Trust, originally thirteen rich, corrupt, and powerful families. Mr. Shepherd, himself a former Minuteman, held the position of Warlord (liaison between the Trust and the Minutemen) and trained Lono and Wylie personally though it is unknown if he trained the rest. The Minutemen were supposed to be killed off on the Trust's orders - instead, their deaths were faked and the members hypnotically hidden in new lives by Graves and Shepherd.
The "hibernating" Minutemen lived very different lives under widely different identities, some going back to the personal lives had before becoming Minutemen while others took on new identities and lifestyles entirely. The Minutemen could only be "reactivated" upon hearing a code-word known by Graves, chosen for its uncommonness and specificity to "the greatest crime in the history of mankind." Additionally some sort of physical/mental trauma seems to play a role in the reactivation of several Minuteman, as some characters are subjected to extreme circumstances as an apparent part of their "waking up." To what exact extent this trauma is tied to the reactivation process (or if it intrinsically is at all) is unknown, as the reader is not explicitly shown every character's specific reactivation process.
Read more about this topic: List Of 100 Bullets Characters