Survivor
Overview
The Survivor is a NPC ally you can find during the mission Find survivors from another expedition from Technician Naylor (CoV, levels 35 - 40). He is the leader of the failed expedition you are looking for. He will be fighting a Chief Mentalist when you find him first, and must be freed so he can help you.
Once free, he will follow you and will attack any Hydra or Rikti in his aggro range. If your char is one of those stealthy types, free him last.
During my mission, the Survivor had slightly less hit points that a Lieutenant of the same level. He was probably a watered-down Boss class ally.
I have no idea if the mission would fail if he'd be defeated. His defense is relatively weak against the Toxic damage the Hydras use at range (like everyone else), but, being the Hydras weak against Lethal damage, his Melee damage output is fearsome, being able to defeat any Hydra he finds in a couple of seconds once he gets in Melee range.
Quotes
During the mission he will say the following:
Fighting Rikti: Lord Recluse will wipe you freaks from the cosmos!
Aggroed: Villain! You gotta get me out of here!
Freed: These things took out my squad. You want my help, you got it.
Information
Crab Spiders function much like SWAT teams in other cities, and are dispatched to deal with high-level threaths like super-types. They're equiped with full body armor, larger weapons and a set of mechanical spider arms that turn them into death-dealing machines.
Powers
During my mission, he used the Crab Spider Webmaster typical attacks:
Powers
Wide Area Web Grenade (Ranged) Foe Immobilize, -Fly, -Recharge
You have been caught in a web. You cannot move or fly and your attack speed is reduced.
Supression (Ranged, Cone) Damage over Time (Energy)
The Suppressive Fire from the Crab Spider is all around you.
Channelgun Ranged, High Damage, Energy, Foe -Endurance
Your backpack has a built-in energy Channelgun which fires bolts of energy.
Slice Melee, Heavy damage, Lethal, Foe -Defense
Your Defense has been reduced by the Claws.
Arm Lash (Melee, Lethal) Damage over Time (Toxic), Foe Slow
The poison on the blade is slowly damaging you and slowing your attacks and movement speed.