The unnamed Horned Reaper Sailor Mercury met first. He has yellow eyes that make him distinct from other horned reapers.
His former Keeper was Metriande, who imprisoned him and left an inactive sub-heart in his dungeon. After a few centuries Mercury appeared and she bonded with the heart. Later he swore allegiance and entered a minion bond with her in the hopes of escaping and more killing.
Due to magic mishaps during Mercury's transformation, he was forced to wear the senshi clothing, an insult he never forgave her.
He grudgingly taught Mercury the basics of dungeon keeping, but let her figure most of it out on her own by trial and error. Thus, among the first rooms Mercury made was his throne room, of which he informed her only after the fact. She promptly collapsed the entry, preventing him from leaving the room.
He was later summoned by his Keeper to fight against the spiders of Keeper Arachne in her dungeon - an order he complied with glee. During that battle he also engaged Baron Leopold, a fight he was on the losing side of and was forced by Mercury to flee along with her.
A botched possession spell by Mercury enforced a body switch, further insulting him. As a punishment for his prior behaviors, he was forced to stay this way longer than necessary.
In his altercation with the baron he blew Mercury's cover, forcing her to relocate. Once she had secured her new dungeon, Mercury opened a portal to the Underworld. He used this chance, severed his connection to her dungeon heart, and attacked outside of her territory. After a close fight, he had to flee, further shaming him.
Mercury imprisoned him, and his last chance to redeem his failures was a duel in Dark God Azzathras name. His opponent, Mercury, won again and in the process he fell into the realm of the dark gods.
Since he has no name, some nicknames for him are Horny and Big Red (neither in-story).
The Horned Reaper is perpetually in the mood to kill, slash and generally harm anyone and anything in his proximity. His sole delight lies in inflicting harm and death on anything close to it. The only time it delays on that is when it plans on means to murder even more people and monsters.
Mercury's senshi transformation forced him into her outfit, including skirt and earrings, greatly infuriating him. Combined with him being forced to inhabit her body for a time, he felt extraordinarily vengeful towards Mercury, which he showed at every opportunity.