Fisheye was one of the Amazon Trio. Unlike Hawkeye who targeted older women and Tigereye who targeted younger women, Fisheye targeted men. Effeminate Fisheye saw Mamoru at a pet store and fell in love with him, or rather his good looks. Here's what happened when he saw his competition:

U: Mamo-chan?
M: Usako?
U: What are you doing?
M: Wait! Nothing wrong, Usako!
F: No make-up, zero sex appeal, fair body proportion... obviously no match for me!
U: What?
F: Sorry, but I got this man.
M: Come on.
U: What are you talking about? Who are you?
F: I'm your rival for love, so to speak! Right! But you're really no match for me.

Fisheye, of course, was determined. One day, he cornered Mamoru and tried to kiss him. Mamoru, of course, wouldn't let him as he only had eyes for his Usako. And this was what they said to each other:

F: I'm not attractive?
M: Yes, you are... But I'm only interested in her.
F: No! Impossible! What do you think is so attractive about that girl?
M: Sorry! I'm really in a hurry!
F: No! No! Because I'm serious for the first time! But tell me! What is so attractive about that girl, Usagi?
M: She has lots of dreams.

(Why Mamoru didn't spout something romantic like "Because she's beautiful inside and out" was beyond me. But anyway...)

The Amazon Trio discovered, courtesy of a smug Zirconia, that they didn't have Mirrors since they weren't really humans. Fisheye was crushed. When Usagi saw him sitting dejectedly in the rain, she approached and invited him into her home (more specifically, her room). Although she knew that he was after her boyfriend, she still treated him nicely. She even lent him dry clothes.

F: I heard that you have a lot of dreams. Mamoru told me so.
U: You saw him again? When? Where? You were wet in the rain because of something relating to Mamoru? Come on! What do you mean by that suspicious silence?
F: You're so easy-going, aren't you? I truly envy you because you have a lot of dreams.
U: Why don't you have a cup of tea? It'll help you warm up a little.
F: Hey, please tell me... What is your best dream?
U: Well, you can't imagine such a simple thing, my rival?
F: Oh, yes. That's right. That is your beautiful dream. Mamo-chan smiles like this when the two of you are alone.
U: Hey! Don't say his name as if you were an intimate friend! Let's make things clear. Don't approach him anymore! I'll tell you that your sense of choice is too good because he's the best man in the world...

What Usagi said made Fisheye realize that she was really Sailor Moon. (You see, in the previous episode, Moon said something similar to that.) He also discovered Chibiusa talking to Pegasus. He left but, unknown to him, Zircon, that freaky winged eyeball, was spying on him.

Anyway, Zirconia had Hawkeye target Usagi's Mirror. And when that Mirror got shattered, Fisheye was shocked. Usagi's beautiful dream was gone! He used the ball of energy meant to capture Pegasus to undo the damage. He died as a result. Of course, Pegasus took pity on him and his friends who also died. Pegasus revived them, gave them their very own Mirrors, and let them live in Elysion.

Btw, the snippets of dialogues above were taken from episodes 146-149.

So you see, though Usagi found another rival for Mamoru's affection in Fisheye, she didn't get all bitchy. I mean, sure she told Fisheye to leave her man alone, but still... If her brief but memorable encounter with Fisheye hadn't taken place, the fate of the Trio would have been quite different, imho. Fisheye wouldn't have had the chance to redeem himself, after all.