Aro Isemi was a Master-candidate of the First Holy Grail War forcibly kept alive through medical technology and sorcery by a faction of magi that survived the previous war. He was a sacrifice who had a beast planted inside him by Manaka Sajyou. According to the 5th novel of Fragments, his full name is Aro Isemi, but prior to this, he was just known by his last name.
Although around for the previous war eight years ago, he currently looks to be a youth of about ten years. Both his hand and feet have rotted away, and all of his organs are mechanical. He is a living corpse whose only living portion is above his breast.
Without friends and constantly suffering, it would not have been strange for him to summon a hero who suffered misfortune as his Servant, but decides against it to summon a "lucky hero", the only whimsical thing he has done in his life. Wishing for a happy hero instead of a tragic one, he believes that the wish of a satisfied man will be a happy one that will warm people's hearts. Despite his suffering, he does not express any hatred for the injustice caused to him due to his angelic character, and he is like a saint despite his body. He lives a life without reward, yet does not blame other and places his faith in the goodness of people. He develops a friendship with Rider and is glad that he has summoned him.
Aro Isemi summons Perseus, knowing him as a hero who had a happy end. Rider is initially shocked by his treatment, having hatred for the magi keeping him alive for their own selfish wish. Rider then develops sympathy that further develops into respect and friendship. Their time only lasts seven days because the summoning exhausts his life. As he dies, he tells Rider his reason for summoning him, hoping that a happy hero will have a happy wish. He uses all of his Command Spells the moment before he dies to grant Rider an incarnated body. He dies smiling, as if seeing a world where all live in harmony.
Rider mourns his death greatly, filling him with rage. He decides his wish will be to grant his Master the happiness of a normal man through the power of the Grail, but his actions become contrary to his Master's expectations. When the researchers grant him a mask to mimic his Master, he slaughters them all, and goes on to slaughter even more for energy, declaring "…For him, for him who wished for people's happiness, –I will kill you, kill you all." He knows that his deeds are precisely the darkest sins that will stain the dream of his Master. Rider uses his last name when posing as a high-school student.
Aro Isemi is a "living dead" Master whose body is thoroughly devastated. His every heartbeat causes pain, and each breath drawn is like swallowing needles. He is constantly drowning in pain without a sense of living. Summoning a Servant is enough to drain his life, limiting his time as a Master to one week before he dies.