Rules for playing FIST.
I (plan to) use the FIST: Ultra Edition rules with some house rules.
Whenever the PCs meet a notable NPC character, we freeze frame on them for a bit, we get a brief description of how they look, their organizational affliction if appropriate and they say something.
NB: If players want a particular NPC to have a freeze frame, they can ask the GM for that. If approved, that NPC becomes notable.
Players can claim that their PC has had a prior relationship of some sort with this NPC (Encountered them in a past mission). The player describes how their PC and this NPC know each other, how they feel about each other and if approved by the GM, this becomes canon and the player gets an extra d6 to use (one time) when performing a check involving this NPC.
NB: If an PC already has a bond with an NPC (from a previous mission), the player can describe this last encounter between their PC and this NPC in order to get an extra dice.
Friendly NPCs can be called in to support the PCs in their missions and will be more likely to appear in subsequent missions. Unfriendly NPCs will be more likely to appear in subsequent missions too, whenever the GM feels like it.
NPCs with relationships with PCs are by definition stronger than their counterparts and thus will be more likely to survive encounters with PCs.
After a mission is over, the GM and the players go over each of their bonds to see if any of them have changed. For better or worse, as bonds change over time whether positively or negatively the NPC will become more and more invested in the FIST operatives actions and seek to help/hinder them in more important ways.
Extra dice are lost when the mission ends, so use them or lose them.
In the mission debriefing, each player can describe a particular event/moment/action that occurred involving a PC NOT under their control that was cool/interesting/whatever to get an extra dice they can spend on an action involving that PC in subsequent missions.