Mars People/Story

Past
The Mars People are the inhabitants of Mars and they have been hiding from us humans in the underground of the planet. They have been secretly planning to invade and conquer the Earth.

Metal Slug 2
By the time of Metal Slug 2, Donald Morden was trying to contact the Mars People and made a pact with them. They helped him try to conquer Earth and they would rule it together. But then, the Mars People betrayed Morden and tried to destroy the Peregrine Falcon group and then conquer Earth. They failed and returned to Mars.

Metal Slug 3
During the events of Metal Slug 3, they returned to abducted General Morden AND one of the Peregrine Falcons, who they wanted to use as a blueprint for a clone army. The Falcons and Morden's army frustrated their plans once again.

Metal Slug 6
In Metal Slug 6, the Mars People joined the Peregrine Falcons to defeat another alien species that feed on the Martians.

SVC Chaos: SNK vs. Capcom
The Mars People also appeared in SVC Chaos and in NGBC. In SVC, they were planning to conquer Earth once again, but the Peregrine Falcons stopped them.

Neo Geo Battle Coliseum
In NGBC, one of them got stranded on Earth and became a waiter in Osaka. WAREZ then invited him to the tournament, promising him a return to Mars as the prize.