Kisenian Hana Yoma
Hana Yoma Kisenian was left on the comet (probably remainders of planet that disappeared) where Fiore found her. Fiore then got the idea (once he picked the flower) that everybody had left Mamoru all alone. That's where Fiore's heart became dark.
Eventually, Sailor Moon showed Fiore the truth about Usagi and Mamoru's friendship, killing Hana Yoma Kisenian.