Several GUMSHOE games offer the players the power to retroactively add contacts and allies into the story. Night’s Black Agents has the Network ability, there’s Correspondents in Trail of Cthulhu, and the GM might offer a contact for an Investigative spend or push.
Some tips on using such characters:
Start with the scenario
If you’re using a published scenario, or have your own detailed plan, then look for existing non-player characters that could be used as contacts. Often, a scenario involves witnesses or other neutral characters – might any of them meet the player’s needs? For example, if the players need some illegal explosives, and there’s a police inspector whose role as written is to be a minor obstacle to the investigators – then maybe a 2-point Cop Talk spend could turn that inspector into an investigator’s old war buddy, who’ll look the other way while they, ah, borrow some equipment from the evidence room.
Using existing characters is sometimes a stretch, but it’s worth it for the sake of narrative convenience – instead of having to inject a whole new character, it feels more natural to use someone who’s already involved in the events of the scenario.
Go for broad strokes
Usually, the starting point for a player-generated character is necessity – “I need an expert document forger/a flying boat pilot/someone who’ll give us a place to hide from the cultists” – and the player has no ideas beyond that concept. Often, it’s sufficient to add a name and some broad character strokes to that concepts – Josef the Paranoid Forger, Alice the Daredevil Pilot, Cyril the Wounded Soldier who has a cabin in the woods. Then ask the player for one thing that most people don’t know about the contact.
Always ask the player how they know this contact. Are they old friends – and if so, from when and where? Relatives? In-laws? Do they know each other through an intermediary? (“The man in the wheelchair sent me to you”). Don’t pass up the opportunity to build a character’s history (and if the player prefers to leave it mysterious, that’s fine too – it’s a choice to be the guy with the shadowy connections and shadowy past).
Complications, not treachery
The investigators can rely on their contacts not to betray them to the bad guys (at least, not immediately). Complications, though, are fair game. Josef the Forger won’t hand the investigators over to the Yog-Sothoth cult, but he wants a favour in return for his assistance – persuade that rare book collector to let him have a look at a particular medieval manuscript. Alice can fly you to the mysterious island, but her co-pilot’s got gambling debts and he needs rescuing from the triads.
Contacts aren’t around for the final scenes
Contacts don’t get to help the investigators in the final part of the adventure. Look for ways for the contacts to drop out of the story (“I’ll keep the engine running, says Alice, you go investigate that ancient temple at the heart of the island”). If the players insist on dragging their tools of narrative convenience into the endgame, then you’ve got free reign to start killing them off or having them turn on the investigators. If a contact’s involved in the climax of a scenario, then they’re doomed to a dramatic ending…