The enemy was the opponent of the Time Lords during the War. (PROSE: Alien Bodies) It was not a species or a political faction as much as a process. It had a name, but the Great Houses were reluctant to use it. (PROSE: The Book of the War)
Before the War, multiple parties speculated about the enemy's identity. Chatelaine Thessalia incorrectly predicted in The Little Book of Absolute Power that the enemy would be mainly motivated by survival or keeping its history intact. (PROSE: The Book of the War) Greyjan the Sane hypothesized that the Enemy were ancestor cells which had been irradiated by temporal interference and energised by a leaking bottle universe. The Eighth Doctor offered the more entertaining possibility that they were "Yartek, leader of the alien Voord, carrying a big stick." (PROSE: The Ancestor Cell) The Book of the War speculated that the increasing paranoia on the Homeworld before the War may have caused it, saying the idea "makes a certain sense, given the nature of the enemy."
In her speech to the Fifth Wave on the thirtieth anniversary of the battle of Dronid, House Military strategist Entarodora said the ruling Houses kept secret the identity of the enemy and its leadership because, if the Houses believed the true enemy was simply a rogue House or a species of time-active upstarts, they would "simply shrug and go back to sleep." Instead, the secrecy made the enemy into monsters, and the subsequent fear would keep the fight alive. The Book of the War posited that the name was kept secret because the "why" of the enemy was more important than the "what". To explain this "why", it linked to entries on the Churchill Index, Immaculata Formosii, the Gods of the Ainu, "Miss Hiroshima", Mohandassa, Sixth Wave Defections, S'tanim, and Violent Unknown Events; however, none of these entries existed in the book, implying that the enemy had tampered with the text.
According to Carmen Yeh's heavily-fictionalised memoir Fantastical Travels in an Infinite Universe, Compassion believed that the enemy was a meaningless distraction and the real threat to the Homeworld would come from within, specifically "House Lucia" or "family". (PROSE: The Book of the War) The Egyptian god Horus described Lolita as a "process" and "a new kind of history". (AUDIO: The Judgment of Sutekh) The Book of the War similarly described the enemy as a process and the War as "a struggle between one kind of history and another." (PROSE: The Book of the War)
The enemy was seemingly bound by the same Protocols of Linearity as the Houses: it was unable to attack vulnerable points in the Homeworld's history, instead encountering the Houses in the same order that it was encountered by the Houses. This was supported by the initial battle on Dronid, in which the enemy's forces were as ill-prepared as the Houses' First Wave.
House Military soldiers used the colloquialism "Rep", an abbreviation of "representative", to refer to the enemy's agents. (PROSE: The Taking of Planet 5) Tonton Macoute once cooked the corpse of an enemy soldier. (PROSE: Tonton Macoute)
In the Mount Usu duel during the filming of Mujun: The Ghost Kingdom, Chris Cwej encountered an enemy agent which apparently summarised its own mystique: "The Scourge. Harvey. Hermes. The coolest character is the one whose face we never get to see." It was represented by the total absence of anything on the recovered film, appearing not as blackness but instead as emptiness and background filmstock. (PROSE: The Book of the War)
On Roma CLII, Marcus Americanus Scriptor killed a creature that was hunting a renegade who had escaped the War by jumping into a parallel universe. The monster was implied to be working for the enemy. (PROSE: Warlords of Utopia)