When Zeromus is first revealed, it takes the form of a fiery spirit on the dungeon screen. Upon entering combat mode, this form is further clarified as seemingly corporeal composed of, or covered by, long blue wisps that resemble hairs or tentacles. Upon using the crystal to expose and weaken it, Cecil and company watch as it transforms into a fleshy and bony organic extraterrestrial being. A mishmash of limbs, claws, bony protrusions and what appear to be entrails, it's difficult to discern exactly what Zeromus is supposed to resemble, if anything at all. Zeromus refers to itself in the plural--"We are evil," it says, in the PSP version of the game.
Within the Final Fantasy IV fiction, Zeromus seems to almost come out of nowhere. While its existence is explained at the tail end of the game, its reason for being is simply described as being the spiritual remnant or reincarnation of one of the game's prime villains (who, himself, is never seen until the end of the game and never revealed as a character until very, very late in the story) and ultimately the culmination of all of that villain's hatred. As the audience, then, we are made to believe that it is the physical manifestation of pure evil and hatred, nothing more and nothing less, and as a result its "motivation" as an RPG villain is one-dimensional.
In all versions of the game, spanning the SNES, Gameboy Advance, Nintendo DS and PSP, Zeromus's combat pattern is founded upon two core attacks.
The first is "Big Bang", an energy attack that inflicts great damage upon all party members (commonly over 1,000 and sometimes over 2,000, depending on what buffs you have cast and what level your party is). In all versions, "Big Bang" also causes a degenerative status effect where your party members' hit-points slowly but continually decrease.
The second is "Black Hole", which acts as a screen-wide Dispel, nullifying all buffs and debuffs currently in effect for both your party and Zeromus itself--effectively rendering moot the sapping effect of "Big Bang".
Throughout the battle, Zeromus will also cast Flare, Bio, and Whirl/Tornado. When you've weakened it sufficiently, it will also turn to casting Meteor, which--fortunately--is not as deadly as it is when it is cast during major story moments. It is also at this point that Zeromus' attack pattern shifts away from using the "Big Bang" and "Black Hole" combination repeatedly.