Diana Stanley in the trenches

This deck is part of a two to four-deck combination, with a William Yorick deck and a Joe Diamond deck and optionally a Zoey Samaras deck; this is designed to be an ideal third member alongside the core of Yorick and Joe. The underlying concept behind these decks is to use Riot Whistle to pass enemies back and forth, allowing the team to continue working to attack the scenario efficiently. In essence, instead of having to pause dealing with the scenario when an enemy appears, the decks use Riot Whistle at the end of their turns to take enemies from other investigators, so those investigators can take their own turns without attacks of opportunity.

Joe Diamond uses Pathfinder to move, and the other investigator(s) use Safeguard to follow him, allowing for efficient movement through the scenario and good use of support abilities while avoiding the risk of any one investigator getting caught on their own and ambushed. Since the investigators are ideally all going to be in the same location alongside enemies, everyone can use Scene of the Crime for maximum effect, and all investigators have On the Hunt to maximise enemy draw. 19 exp has been chosen as a good break-point, especially since this is more likely to be a fun one-time gimmick for a standalone scenario.

Diana's job in the team is to provide flexibility and additional clue-gathering, and to take on treachery cards for the team with her cancellation effects. Ideally, she wants to get Riot Whistle, Twilight Blade and Trench Knife out as soon as possible, and buff up her willpower to investigate with Sixth Sense, while also being willing to reuse her cancels to keep the team going. Summoned Hound is usually a liability, but in this team it's much less of a problem since juggling enemies and mitigating attacks is the core idea behind the combination - in some situations it can be actively beneficial to draw Unbound Beast, as it enables Scene of the Crime when there's no other enemies in play.

This deck packs a huge array of encounter manipulation and mitigation events, and is broadly designed to let the rest of the group do their own jobs as smoothly as possible. If additional combat ability is required (particularly if not playing 4-player with Zoey Samaras), this deck could be tweaked to include Spectral Razor and/or Azure Flame.

Upgrades from here include Defiance (2), Guts (2), Safeguard (2), and Scrying (3) in place of First Watch, as well as Blood Pact (3) and Deny Existence (5) if you're feeling particularly fancy.

I think Diana is best suited to this role, but it could also be a good option for Sister Mary with basically the same deck (merely replacing Sixth Sense (4) with Rite of Seeking (2), removing a few events and probably adding in Ward of Radiance, Rite of Sanctification and Radiant Smite), using her Bless-adding ability to provide support for the team as a whole - if doing so, the other investigators could also be tweaked to use some more Bless effects (e.g. Righteous Hunt for Zoey and Beloved for Yorick).