Avenger (Fate/hollow ataraxia)

Avenger is the eighth servant class, whose true identity is initially believed to be Angra Mainyu, the spirit of pure evil that was accidentally summoned during the 3rd Holy Grail War and had been imprisoned within the tainted Grail ever since. He usually has a lazy and laid-back attitude, lacking any sense of justice.

Abilities
As a combatant, he seems to be the weakest of all Servants in every parameter, although he can be quite agile and relentless. His fang-like twin daggers Tawrich and Zarich can be used as sword breakers, yet are unreliable as they sometimes break themselves, and his skill in manipulating them as melee weapons is only average compared to other servants.

His battle-oriented Noble Phantasm is Void Avesta; from the Book of Zoraster. It is described by Saber as a 'Wound-Sharing Primal Curse,' as it causes the inflicter to feel the physical effects of the wounds that Avenger takes in battle once he recites it's name. Avenger states that the wound is written on the soul of who he casts the curse on, and regular healing is rendered useless as it won't disappear until either the wound on Avenger heals, or Avenger dies. This Noble Phantasm isn't considered fatal in most cases, although can severely cripple servants; and can likely kill weaker humans. In addition, Avenger must reciete the name of the curse each time he wants to apply it's effect, and this can only be done when he is wounded. Avenger uses it as a chance to weaken and push Saber towards using Excalibur, so that Bazett could have a more even match and finish her off with a counter attack from Fragarach.

True Identity
Avenger was initially believed to be the Persian God Angra Mainyu. In reality, he was no more than an ordinary villager. One day, he was randomly chosen as the source of all evil in the world by his village and was tortured and sacrificed in the following rituals. The villagers believed that the only way to realize the true goodness of human beings was to put all the evil onto one person and blame him as the true source of any human evil. His forced sacrifice eased the confused minds of his people, which despite being unintentional, qualified him to become a Heroic Spirit. However, Avenger himself only feels anger and hatred towards the world and the people that mercilessly executed him.

Yet as the time passed by and he saw many rises and fall of generations, he realized that while he still hated humans for what they did, he still loved the world. In the end, he accepted his role for humanity, even if it meant to be the blame for their wrongdoings.

Role
During Fate/stay night, Avenger was trapped in the wish-granting spiritual Holy Grail, and his influence was the one that tainted it. After falling during the 3rd Grail War, he was trapped in the Grail. Despite having been a normal, powerless being prior, the wish engraved in his soul by those who desired him to represent all the evils in the world was granted. Thus, the purpose of the 4th War was simply to gather vast amounts of mana to grant the wish. However, his emergence was prevented by Emiya Kiritsugu, who commanded his servant, Saber, to destroy the Grail upon recognizing the evil presence emanating from it. However, during the 5th, he was finally reborn as the Grail, thanks to its immense mana. Yet the physical vessel for the Grail was destroyed by Emiya Shirou to prevent the disaster to be brought by the presence of Avenger. After finally being able to grant a wish, the Grail was left to expire without accomplishing its role. In order to do what it was born to do, Avenger/the Grail sought a wish. In the process, he found Bazett, who after being left to die, wished hard that she did not want to die. Therefore, they made a contract: Avenger, to continue to exist and accomplish its new role of granting a wish, and Bazett, to stay alive even in a vegetative state.

However, 6 months after the 5th war, Caren came to Fuyuki city, found and nursed the dying Bazett. Unlike before, when Avenger just had to keep her physical signs alive, now he had to keep both Bazett's now awake consciousness and body alive. In order to accomplish that, he created the four-day temporal loop.

To keep Bazett thinking that she was alive, Avenger recreated the 5th Grail War. The problem, however, was that he never personally experienced the 5th war. He only knew of the 3rd war. So he just took the participants of 5th war and used them to create the characters in the 3rd war to play the 5th war events.

While he was Bazett's Servant Avenger by night, he took on the form of Shirou by day. As Shirou, he comes to care for other people, especially the mysterious Caren Ortensia. In this form, he has no idea of his own identity and thinks and acts like Shirou. He later explains that this is because Avenger himself is 0, and no matter how much you add 0 to 1, it is still 1.

The problem starts when (1) Shirou, in truth Avenger, realizes the temporal loop and tries to solve the anomaly, and (2) Avenger himself begins to keep "filling up" the Grail to see more and more new events. Since Emiya Shirou is someone who cannot let the wrong stay wrong, he had to fix the abnormal temporal loop. Also, even though he knew that by seeking new events he would complete the Grail thus ending the loop, which would ultimately mean his return to void, Avenger could not stop enjoying the happy daily life as Shirou.

Later Bazett realizes the false reality of the temporal loop, but refuses to finish it. She thinks that she would die once the loop is over, but she also knew that would permanently take Avenger to nothingness. In her hope to stay alive, and her wish to keep Avenger from his ultimate fate, she tries to continue the loop, even if it means going against Avenger himself.

With help from Caren, the only one whom Shirou has never met and thus Avenger reveals his true self, Shirou/Avenger finally realizes that he himself is the cause of all the problem, and decides to put the end to it. He finally reaches the Grail as the other Servants and Masters prevent the monsters(which are actually leftover parts of Avenger) from stopping him. In the end, Avenger frees Bazett from the loop and returns to the void.