The Melee class is both the sturdiest and most agile combatant, ready to dash directly into hand-to-hand combat and inflict punishment as well as absorb it. Another strategic use of a Melee combatant is to treat them as a human shield for more vulnerable Rifle crew members. They are also extremely useful at killing Deadeyes, as their huge movement range and Deadeye's low movement range combined with the fact that deadeye's can't shoot near them is a great combination. You should always have 1-2 Frontliners in each fight.
As the close-combat weapon specialists on the Dozer crew, the Shotgun class is fairly resilient and packs whopping firepower when the enemy is near, but is much less effective at a distance.
The Pistol class is a great balanced soldier with moderate endurance, movement and weapon range. These hand-cannon hotshots are also the only class with run-and-gun ability.
The long-distance engagement expert in your crew, the Rifle class has the lowest health and movement range of your crew, but also deals the greatest damage at the furthest distance. These sharpshooters are integral to any good battle plan and should be protected!