The base Ranger has a stealth melee tree and a gunner shotgun tree. So to keep in that spirit, the Dark Ranger is based upon the Assault Infantry while utilizing skills from the Marksman's stealth tree to maximize the Run & Gun and melee nature of the Ranger.

The base Sharpshooter has a sniper tree and a pistol tree. So to keep in that spirit, the Dark Sharpshooter is based upon the Marksman while utilizing skills from the Marksman's holotargeting and sniper trees to maximize their long-distance warfare nature.

The base Grenadier has a cannon support tree and an explosives tree. So to keep in that spirit, the Dark Grenadier is based upon the Sapper while utilizing skills from the Marine's gunnery trees to maximize their explosive nature.

The base Specialist has a healer tree and a tech/hack tree. So to keep in that spirit, the Dark Specialist is based upon the Technical Specialist while utilizing skills from the Field Medic's healing trees to maximize their gremlin-based combat.

The base Psi Operative has skills around crowd control and heavy direct damage. So to keep in that spirit, the Dark Psi Agent is based upon the original psionic class while utilizing the Field Medic's healing and combat skills to balance them out.