Mamoru And Fiore
We are introduced to Fiore in the Sailormoon R Movie. He mysteriously appears at the beautiful gardens Mamoru and the girls are visiting for the day. In a flurry of wind and petals, this mysterious brown-haired, odd-looking boy around the same age as Mamoru appears. He seems to know Mamoru very well; Mamoru, on the other hand, seems puzzled and searches in his mind to think who this person who refers to him as "Mamoru-kun" could possibly be.
He apologizes to Mamoru for "taking so long" as he approaches him, then takes Mamoru's hand in his, to which both Mamoru and Usagi seem a tad uncomfortable about. Usagi chides in and gently tries to explain that he can't take Mamoru anywhere because he's her boyfriend. Fiore doesn't like Usagi's answer, and decides the best thing to do is to shove Usagi to the ground. Fiore then proceeds to get yelled at, but all he really seems to focus his attention on is Mamoru. He tells him that he'll be back, and disappears. Mamoru says to himself, "Fiore...?"
We later find out that right after the car accident that killed Mamoru's parents, Mamoru found Fiore outside the hospital in the rain. He brought him back inside, where they became friends. Since Mamoru had grown up, he had totally forgotten about the encounter that occured when he was six years old, and even dismissed Fiore as being something he had made up when he finally did remember him.
However, Fiore wasn't a dream at all. Quite the contrary, he turned out to be a nightmare for the sailor senshi when he returned. Luna and Artemis revealed that Fiore was being controlled by the Kisenian Blossom; a rather evil flower that latches onto and controls people with weak hearts.
Fiore was so intent on returning the favor of a flower that Mamoru had given him when Fiore left the hospital all those years ago. At first, he thought that the Kisenian was the perfect flower - but it took over his mind and heart instead. Fiore's gratefulness to Mamoru for being his only friend turned into something of possessive jealousy. He hated Usagi because Mamoru paid heed to her, and was in love with her.
Is Fiore Gay?
I see this brought up time and again. After the incident at the gardens, the girls joke that maybe Mamoru and Fiore are secretly gay. While Fiore's actions were very strange while under the influence of the Kisenian, I never really thought that he had a strictly romantic love for Mamoru.
Fiore related to Mamoru because when they met, they were both very young, and very lonely. Fiore had been wandering the galaxy alone, looking for some place to stay. Mamoru, on the other hand, was lonely because his parents had just been killed in an auto crash, and in the process he lost his memories. They were both very lonely and confused children who needed someone to keep them company and to confide in. They found that in each other.
But, while Mamoru moved on as he aged and made friends, and of course met Usagi, Fiore was still somewhere out there alone. All he really had were the memories of his only friend, Mamoru. While Mamoru was able to escape his loneliness, Fiore was not. He was easy prey for the Kisenian.
Fiore, when he finally found Mamoru, was under the control of the Kisenian, so I personally think that this caused his loneliness to escalate into contempt for those around Mamoru. I believe this is why he hated Usagi when he met her. Not because he wanted to be Mamoru's boyfriend, but because, in his controlled state, he felt anger for these people. He wanted things to be the way they were when he had first met Mamoru. Just the two of them, friends who understood each other's situation. Fiore felt that Usagi and the others couldn't really understand neither him nor Mamoru.
He captured Mamoru because he thought that things could be the way they were. At the same time, he was going to fulfill the Kisenian's wishes of destroying the Earth. In the end, Fiore's true nature came through with the help of Usagi's understanding, and the Kisenian was destroyed. Note that at the end of the film, Fiore is more than willing to help Tuxedo Kamen revive Sailormoon, who has just died by using the ginzuishou. He then goes off again to start anew. Maybe it really was the intentions of the writers for Fiore to be homosexual, but I just never really saw it that way. In the end, Mamoru and Fiore were two alike souls, even though one was human and the other was not. I'm sure that each was thankful for the other's company at such hard, lonely points in their young lives.