POWs (or Prisoners of War) are non-playable characters from the Metal Slug series.
Rescuing them often yields various bonuses and can be anywhere on the level, some even hidden from view. Some POWs even lend a hand to aid the player, which is mostly directly engaging the enemy.
All remaining rescued hostages count towards the final mission score at the end but the counter resets to zero whenever the player is killed.
Prisoners of War These are the most common POWs throughout the entire Metal Slug series. When rescued, they pull out an item from their pants to aid the player. Such drawn items include special weapons.
Rumi Aikawa: A wandering absent-minded supply sergeant with a backpack, packed full of goodies. When the player encounters her, she provides an item. If the player damages her backpack it will drop various food items.
Madoka Aikawa: Rumi's sister. Same job, same role, same mindset.
Sailor (unused): Unused POWs from level 5 of Metal Slug 4, these POWs are in a Cargo ship overrun and captured by Amadeus troops and Pirates. The idea of them being prisoners was scrapped and instead they are only seen running away and jumping overboard, like civilian NPCs in urban levels.
Executives: These POWs act the same way as the regular POWs do. When rescued, they each give one random item from briefcase. These POWs can be seen only in Metal Slug 5.
President: A POW that looks like a caricature of Bill Clinton. When rescued, he calls two bodyguards to each provide an item for the player. He also provides a higher bonus score at the end of the mission.
Regular army recruit: These POWs are essentially the same as regular POWs. They provide the player one random item from their backpack. These POWs replace normal POWs for the first mission in Metal Slug Advance.
Hyakutaro Ichimonji: A 2nd lieutenant who takes disguise as a POW. When rescued, he supports the player in combat by charging a projectile similar to a Hadouken (but with a burst similar to Andy Bogard's Hishouken). He also performs melee attacks with a spin kick. He first appears in Metal Slug 2.
Pupipi: A Martian POW who supports the player in combat for some time with a mini-UFO. He shares the same role as Hyakutaro Ichimonji. He is seen only in Metal Slug 6.
Acchiles (unused): An unused POW from Metal Slug 5, originally intended to support the player in combat with exploding machine guns, accordingly with the fan site Metal Slug: Missing In Action (which also began the trend of calling him "Glen Achilles", only found in a unused arena of Mission 3 from Metal Slug 5, along with Flame Shot).
Frozen rebel soldier: When rescued, they count towards the player's bonus score at the end of the mission, but do not provide any items.