Beautiful...and handsome Seiya Kou in the manga was a woman who disguised herself as a man. In the anime, she and the other 2 Starlights actually transformed into men while on earth but then returned to their female form after they found their Princess Kakyuu.
So what's all this about Seiya and Usagi as a couple? Did it actually happen? Read on and find out.
"I can't imagine why people would want to put Seiya and Usagi together - not when Usagi's so obviously meant for Mamoru. I mean, Seiya's a nice person, romantic and beautiful and powerful... but she and Usagi don't match the way Mamoru and Usagi do. Besides, how can Seiya ever devote her heart to Usagi? She has to be loyal to her own princess first; that's her duty as a Kinmokusei senshi. Mamoru's heart and soul have only one person to protect: his Usako-hime."
Indeed, it is hard to understand how one could wish Usagi to be with anyone else but Mamoru but in the character Kou Seiya, we find several reasons. Ilsevet has provided below information on the manga about Seiya and I have wrote an article based on it.
For starters, the anime and the manga Sailor Moon are very different. The manga is the original and it portrayed Mamoru as loving, sensitive, and tender. In the manga, he shows his love for Usagi repeatedly and he never ever broke up with her. His breaking up with Usagi in the anime was something the animators made up and was never part of the original story line.
The anime then made Mamoru into a cold, rude jerk in the 1st season and most of the 2nd, and because of how he was portrayed in the anime, some fans felt Usagi should be with Seiya. They seem to forget all the times in both 1st, 2nd (R season) and S and SuperS seasons where Mamoru was very sweet and loving to Usagi.
This article is here to show you just WHY Seiya would not be a good match for Usagi. We begin with the manga.
Manga Volume 16: Mamoru has just asked Usagi to marry him, and gives her a ring. Usagi cries tears of joy and both kiss pasionately. Suddenly, out of no where, comes this woman, Sailor Galaxia, the main villain of the Stars series. This villain takes Mamoru's Golden Crystal - his star seed/what is his very essence - and in doing so, kills him. Usagi watches with horror as her beloved disappears, his last word being 'Usako'. Usagi faints and who catches her? A beautiful stranger named Kou Seiya.
Seiya is a woman from the Kinmokusei Solar System who came with her fellow Kinmokusei Senshi, Kou Taiki, and Kou Yaten to find their Princess Kakyuu. This Princess had fled her galaxy to run away from Sailor Galaxia who was out to steal all the universes' Sailor Senshi Crystals. Seiya now has witnessed Mamoru's death and takes an unconscious Usagi home.
Seiya, from that moment on, notices similarities between Usagi and his Princess Kakyuu, and begins to fall in love with her. Seiya disguises herself as a man on Earth - she is still a female under her clothes (in the anime her body changes into that of a man on Earth) - and befriends Usagi who has blocked out Mamoru's tragic death from her memory. Instead, she thinks that Mamoru got onto his plane safely and arrived in America where he was to study at Harvard. She is very alone and feels sad wondering why Mamoru does not write to her as he promised he would. She really cannot remember that he died right before her eyes.
Seiya feels things for Usagi at first because Seiya was actually in love with Princess Kakyuu - she sings a song for Princess Kakyuu (in both anime and manga) in which she speaks of a love she has for this person - and this is before Seiya really falls for Usagi.
The similarity between Usagi and Princess Kakyuu is the smell of olives which Usagi has gotten from an incense lamp in her room. Princess Kakyuu is from a planet of olives and always smells like that. It turns out, Princess Kakyuu is IN the incense lamp and that Chibi Chibi is the one who placed the incense lamp in Usagi's room so Usagi would rescue Princess Kakyuu. This smell of olives is what helped Seiya into viewing Usagi as the Princess Kakyuu she loved.
Seiya realizes though, in both anime and manga, that he/she is not the one for Usagi - that her heart has belonged to Mamoru from the very beginning of her existence, and always shall. In Act 46: as she watches Usagi walk away, Seiya thinks to herself: "We are of different stations in the universe. This might be an unrequited love." And unrequited love it is. Usagi never loves Seiya back in that way and is always loyal to her Mamochan. She sees Seiya as only a friend, and nothing more.
Besides Seiya's own realization, and the fact that Usagi would love and choose Mamoru always, there are other reasons why a matching between the Fighter and the Moon would not work out.
For one thing, there's the matter of the long-distance relationship. Seiya lives very far away; if Usagi finds it hard to part with Mamoru when he goes to America, how would she feel when Seiya must return to Kinmoku Star? And don't tell me that Seiya will stay on Earth because of her love. Seiya will never stay with Usagi. They are both loyal, dutiful senshi; Usagi will remain with the Moon, and Seiya will go back to Kinmokusei and serve her princess.
That's another point - Seiya already has a princess to protect and defend, and it's not Usagi. As a Kinmokusei senshi, Seiya must devote herself to protecting Kakyuu-hime, just as Mamoru and the senshi of our solar system devote their very lives to protect Usagi.
Mamoru, on the other hand, has no other calls on his loyalty. He has only one person he must protect: his Usako. He has sacrificed his life for hers several times before, throughout the centuries; he will not hesitate to do so again. At one point, he even cries out that Usagi is what keeps him alive. How can Seiya, a senshi with another princess and a duty far away, ever match that kind of loyalty?
And if you want to get technical, there is no match for a princess but a prince of equal rank. Mamoru-kun's Golden Crystal is the only perfect match for Usagi's powerful Silver Crystal. Seiya's Kinmokusei sailor crystal isn't even in the running.
Lastly, the most important thing to remember about Seiya is that he/she and Mamoru had NO ill-feelings towards each other. In both anime and manga, both only had respect for the other. The way they interacted in the final episode 200 of the anime shows a Mamoru who appreciates all that Seiya did and shows a Seiya who knows Usagi will be loved and safe always in the arms of her Prince.
It is easy for Usagi and Mamoru fans to get angry with Seiya and Usagi fans but what we must remember is that Usagi chose her man. She chose Mamoru. She was face to face with Seiya, was kissed by him, was shown Seiya's affections and yet Usagi was never won over - she never returned Seiya's feelings or ever thought about Seiya in that way. To her, Seiya was always a friend, and Mamoru was always her one true love. Usagi and Mamoru, who have faced challenges from other potential lovers, to enemies, to evils and death have overcome it all. They have remained true to each other to their very deaths and even beyond death. No matter how much some fans may believe Seiya should be with Usagi, the fact of the matter is, Seiya - noble and worthy candidate as she was - did not win the heart of our Usagi.