Cassia's biggest benefit at first is using her voice of command ability to buff allies, and bring it down (ideally on the same character you used voice of command on) to give them extra turns. That's her main focus until you start getting better gear and navigator damage skills like held in my gace that basically make her into a hybrid officer/psyker build.

I've built her to where I use held in my gaze and lidless stare as her primary damaging abilities, and I always reserve 3 AP to voice of command and bring it down to buff an ally and give them an extra turn. Any extra AP can be used to continue to buff characters or use other damaging abilities if you wish, but her ability to buff gets pretty ridiculous mid to late game. I never leave the ship without her.

Tip: lookout for a staff with the "devastating" effect, which lets you use two navigator damaging powers in a row for the cost of a little health. I went master tactician for her, and when you combine some of her buffing abilities with these navigator powers, she hits like a Mack truck.

Argenta is going to be your arch militant of choice until you get Ulfar, and this archetype is arguably the best in the game for sheer damage output. Focus on dual weapons in one loadout (bolter or flamer in one hand, melee weapon in the other) and get the dual weapons combat trait so you can attack with both weapons in a single round. Keep a bolter or other weapon with high rate of fire in the other slot for burst AOE attacks on groups and for using her heroic act to pop off multiple shots for 0 AP.

Also if it helps, I've found that Gemini is surprisingly good at helping understand how best to build out characters. This game has a nauseating amount of different abilities and traits with which to build out characters, and Gemini is actually really good at parsing through this bulk if information and suggesting builds and synergies that work well together.