Cancer is the main enemy of the game.
Details[]
Cancer is described as an invasive extraterrestrial species of creatures that comes to the Earth via objects that can be described as "asteroids", and invaded most of it since. The Cancer name is given as they are deemed cancerous to the Earth.
Most Cancer can only roam on land, they can not fly or swim even though they do possess ability to jump high. This means for some while, smaller island regions like Japan are not exposed to its danger. But that does not mean those are entirely safe, as troops of Cancer can continuously land on Earth, and some can project their attack on a distance.
Cancers are invincible against ordinary man-made weapon, and only Seraph could harm them for now.