I use the term "Frankenstein's creation" because in the manga, Fish Eye was once a happy little fish in the Kyoto River. Until PallaPalla found him. She turned him into a human and ordered him to find the dreamers of beautiful dreams from the White Moon Kingdom. His target was Ami, aka Sailor Mercury.
PallaPalla set up a store where she sold fish. Ami wandered in, and wanted to buy a fish. PallaPalla recommended Fish Eye, who was in his fish form. He reminded Ami of one of her father's postcards, so she bought him and took him home.
Ami sits at home, depressed. She looks into the mirror and sees the happy family of Usagi, Mamoru, and Chibiusa (webmistress comment: happy? 0_o). She sighs. That's the family that she longs to have.
Fish Eye transformes into his human form. He asks Ami about her dreams. When Ami reveals that she wanted to be loved, Fish Eye started kissing her (webmistress comment: should've been Mamoru. GRRR).
Ami resists Fish Eye, and realizes that she's trapped. She falls into this strange dream sequence. Fish Eye is happy at his success, but Ami sees Sailor Mercury, who pulls her out of the dream sequence and gives her new powers. Sailor Mercury, Sailor Moon, and Sailor Chibi-Moon destroy Fish Eye.
I prefer the Anime Fish Eye, because he is much more wacko and less of a minor character. But that's just me.