There are four Krusnik: Cain, who is Krusnik 01; his younger brother Abel (02) and baby sister Seth (03); and Lilith (04). All are test-tube babies, specifically created to be the leaders and defenders of a Terran colonizing expedition to Mars ("the Red Mars Project"), and hold miltary rank in the United Nations Air and Space Force. They are collectively referred to as the Night Lords, and each has taken a version of that as her/his surname--Seth Nightlord, Abel Nightroad and Cain Knightlord. Each one has a major role in the events of Trinity Blood.
Abel and his siblings originally shared a bitter grudge against the humanity which had created them as mere tools and watchdogs, and returned to Earth among the second wave of Returners from Mars, which intended to conquer the planet. (Read the section "What are the Methuselah?" for more on the Red Mars Project and the Returners.) Their Krusnik powers gave the invasion a tremendous advantage, and humanity was on its knees. Lilith--Abel's beloved, and the fourth Krusnik--was outraged by their actions and decided to go to Earth to fight on the Terran side, bringing them technological secrets that had not been released to mere humankind. With her help the tide of battle was turned, and at a major battle in Paris, Lilith and the combined Terran forces brought the Night Lords' army to a standstill. At this point, Cain--who had long since permitted full 100% release of the Krusnik nanomachine in himself and allowed his mind to fuse with it-- requested a truce and a diplomatic meeting with Lilith to discuss terms. She returned to Mars to meet with him, only to be betrayed: Cain murdered her and claimed her technical secrets for himself.
Lilith's death drove Abel temporarily insane. He battled his brother and literally threw him from space to Earth, then took Lilith's body and presented himself at the Vatican, begging for sanctuary and a chance to atone. This essentially brought the war to an end in a single day.
The Vatican took Abel in and allowed him to bury Lilith in an underground catacomb, which he has guarded for centuries. Having seen what happened to Cain, he has never even once allowed 100% release of the nanomachine in himself. In his heart he still hopes that removing the nanomachine from Cain would restore his brother to his original self (he's said to have been a very kind and intelligent person and an excellent commanding officer, loved by his troops as well as his siblings) though realistically he knows it's not likely.
Seth, realizing from this that the Krusnik had completely destroyed the big brother Cain she knew and loved, likewise called truce with humanity and devoted herself to building a separate but peaceful empire on Earth for the Methuselah. Cain, a shattered ruin, was found by the Rosen Kreuz, which built a body for him using high magick/technology and gave him a vehicle to use in his now implacable goal of destroying all Terran humanity --and his younger brother.