The enigmatic entities which wish to wipe out all of humanity.
The first Jotun appeared in Asgard 04, where the great was first discovered and assessed. The beast was unphased by conventional warfare, and the military was forced to resort for use of an N2 mine. This slowed the monster down long enough for NERV to deploy its own secret weapon, the Aesir, which after a prolonged battle, managed to reign victorious.
The ODIN Systems has analyzed the data collected from that first battle and theorized several universal factors, at a high percent chance of them all being true of the species as a whole: they are all colossal in size compared to humans, they all have no vital organs save for a red sphere known as a core from which the rest of their mass originates, and they have unparalleled mastery the Absolute Territory Field. Many of their abilities defy current mathematical and scientific explanation. Whether this is the extent of their abilities of not is yet to be seen.