Outside of models I've truly never used before, there are some were I have to congratulate Wyrd on the flavor and abilities of when it comes to their models. Being the mainly Arcanist player I am, the first thing to come into my head when thinking of impactful non-Masters is the Coryphee and the Duet they become.
Back when I first started Malifaux, I had a very real problem with Scheme runners with Rasputina. With little experience with the Ice Witch and at a time where not even all the masters crew boxes were released, it was hard to generate a list for Breakthrough/Plant Evidence etc etc, Frozen Heart back then just didn't have any runners.
My first inclusion were the Mole Men and for a while they were great, that is until they were attacked or, for instance, they had already dropped more than enough markers. They have a Very focused job and can't really diverge from that role, I needed something else.
As a new player I brought both Arsenal decks for Arcanists just so I knew what everything did (This also being when Crossroads wasn't released, dark times indeed) and the Coryphee appeared to be exactly what I was looking for: Fast models who were able to attack but more importantly could leave combat and not get charged in response. The Duet looked insane and rightly so for sacrificing two 7ss models. So I set about a proxy of the dancers and took them to the table.
... And subsequently got them killed before turn 3 every game I had them. As a player in general I have this idea that new models exist in another dimension where opponent's will let them do exactly what they want and watch in awe as they do their home run lap. I can't fault my opponent's actions, I was playing the Coryphee dumb. I ran them in range of Anti-Armor which destroyed them quickly, if I had planned to run the pair separately, I would panic at the loss of 2 wounds and try to duet them only to have them standing out in the open. In games where I duet'ed immediately I would use them as pure scheme runners rather than assassination units so spending 14ss to be backfield doing Mole Men's job (forgetting that with their speed and 3ap, they could achieve a number of base rulebook schemes on turn 5).
I would either play too cagey or too aggressive, but as I came to learn, there is a delicate nuance of balance to the puppet's role on the field.
Fast forward to modern day, the Coryphee are released in plastic now and by budgetary reasoning, I've magnetized them to properly duet together. I know now not to run them into big nasties but rather assassinating smaller models. I almost never run them as a single unit anymore, the first turn is hopefully a double walk into a Dance Partner-Dance Together. Why not duet immediately? Summoning a fresh model that still has 2ap left to use up to 17" into the table is veritably useful, especially in dealing with well-placed From the Shadows models or taking down an early minion who safely assumed double walking was a good move. Picturing two marionettes gracefully spinning across the battlefield and effortlessly taking down a Guild Pathfinder is a thing of beauty.
I have found despite this that Rasputina still doesn't like the Duet. Not being Frozen Heart and not having much synergy in the crew leaves the puppets in the box. Mei Feng on the other hand loves them. Fast constructs that can keep up with her, that can get her out of trouble via Rail Walk and being able start or finish a fight alongside the Rail Boss is amazing. The Duet's main weakness against range becomes silly when they are within the protective Vent Steam and having another 3ap model dancing beside a master and the often 3Ap Firestarter provides fantastic opportunities for Vp gain.
With only one game with Kaeris right now I don't know if they have a place but in their one game with her they provided a means to draw an optimal hand turn one before duet'ing and acted as bodyguards to the rather frail Kaeris saving her Ap to use her burning effectively.
Judging the Duet on it's own, having Auto-everything in attack and defense paired with Agile and Disguised make the dancers impossible to pin down with melee attacks alone. Hitting them with conditions or ranged attacks is preferable but to get through Df/Wp6 with a built-in positive, it's an uphill struggle. This is not to say they are invincible, far from it but so long as I keep them away from tables that can focus anti-armor rounds, avoiding charging them into more than 2 models who can wear down those 7Wd OR being in a position where models around the Duet prevent them from escaping then I'm confident they will always make for an memorable game and an additional challenge to my opponent. It's 14ss that I don't regret spending to include them.
Dance like no one is watching and have fun out there.