He was the Master of Rider in Fate/Prototype. His every heartbeat causes pain, and each breath drawn is like swallowing needles. He is constantly drowning in pain without a sense of living. In Fate/Prototype: Fragments of Sky Silver

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Aro Isemi was a Master-candidate of the First Holy Grail War forcibly kept alive through medical technology and sorcery. He was a sacrifice who had a beast planted inside him by Manaka Sajyou.

In Fate/Prototype

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Aro Isemi summons Perseus, knowing him as a hero who had a happy end. Their time only lasts a week 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.