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Tear Jerker: Psychic Academy
Ren when he's Killed off for good, and tells Fafa not to cry anymore.
Ai is forced to view his memories...including his true parents' deaths.
Gyurazo's special ability is to drain the auras of others, indirectly causing his sister's death, and causing Ren to be Killed Off for Real. This becomes a little more weighty when you consider his primary motivation for honing his Aura Powers is to cure his Ill Girl sister.