The other characters are solidly placed in the narrative and are all working through conflict and a character change in their own ways. Katie especially, she's hiding a massive secret from her husband and family at the same time as trying to keep hold of the tiger's tail that she's grabbed with her involvement in The Resistance.
The relationship, and the hidden conflict between Will and her husband is one of the central themes of Colony - it's not hard to see why the characters are there.
Will is a professional and doing a good job, but he's doing it for what he knows is the wrong side, the side that killed all the politicians, law enforcement and military as soon as they arrived. I wouldn't mind seeing a bit more angst and worry about what side he's really on from Will, but I get that he's only doing it to get his son back.
Eric Broussard - a nicely drawn character, it would have been so easy to portray him as a flat, boring, soulless killer. But the show has enthusiastically gone the other way and shown him torn between his instinctive (and correctly placed) trust in Katie and his desire to follow orders and get the job done.
Alan Snyder, is just a masterpiece. What a self serving rat he is - but even he's not just a baddie, we sometimes get glimpses of an actual person in there, when he talks to Katie about New Orleans for instance.
Maddie though? The character is very little used, she rarely appears at all and her story doesn't seem to interact with the story-lines of any of the other protagonists. I know she lives in the Bowman's converted garage and that her son is schooled alongside Gracie Bowman - but that's it. Apart from those two elements, and even those haven't been explored by the story, she might as well be a separate series, Colony: The Maddie Kenner Story.
Maybe she's going to get herself involved in something actually political with one of the Burgesses... I hope so, at the moment she seems "underused", to put it politely.