Immortal NPC

Some NPC allies in Earth Defense Force 5 have the special property of being immortal. While a regular ally NPC reduced to 0 health will die and their corpse vanish after a short time, these troops behave differently.

General
When reduced to 0 health, an immortal NPC will drop to one knee in the place where they "died," but will not disappear. Instead. they will remain in place until a condition change occurs in a mission (a new wave appearing, for example) and then will get back up with 20% of their health restored.

This mechanic is presumably to deal with the issue in Earth Defense Force 2025 and Earth Defense Force 4.1: The Shadow of New Despair where NPC dialog would not play if the character who was supposed to say it was dead, sometimes resulting in important or silly dialog being skipped.

Alpha Team
Alpha are a four-man Ranger team led by the Sergeant. Typically they carry standard Ranger assault rifles. Towards the end of the game the Sergeant will frequently turn up to missions with his "borrowed" Blazer in hand, making him an extremely valuable asset. When HQ assigns Storm Team labels to the EDF unit assaulting Base 228 late in the game, the Sergeant (though seemingly not the rest of Alpha) is assigned the code "Storm 2."

Grim Reaper Team
Grim Reaper are a Fencer team led by a character simply called "Grim Reaper Captain," later becoming "Storm 3." They are equipped with Shields and Blasthole Spears and make aggressive use of their dash thrusters, typically going after anything which passes within about 80 metres of them if anything on the map is aggro'd. Grim Team varies in size depending on the mission, from as few as three members to two squads of five members. When present in multiple squads, Grim has a subordinate Captain leading the second.

Grim are the team least likely to be immortal: often only the Captain is, and sometimes not even him.

Spriggan Team
Spriggan are an elite Wing Diver unit with the unimaginatively-named "Spriggan Leader" in charge. The unit is later labelled "Storm 4." They are typically armed with Mag Blasters and Power Lances. Spriggan is usually the smallest team, sometimes with as few as two members.

Spriggan are also less likely to be immortal than Alpha, though more commonly so than Grim, and like Grim, often only Spriggan Leader is flagged as immortal.