Seiya and Usagi

Their first encounter was at the airport (episode 173), when Usagi is seeing Mamoru off. He's studying abroad, and he's just proposed to her. Seiya had just landed and while wandering away from screaming fangirls, he walks past Usagi. While Usagi didn't take notice of him, Seiya sensed a strong mysterious power surrounding her.

The following day he sees her again at a park, though she doesn't recognise him which surprises him, since he is the lead singer in the popular idol group Three Lights. Not long after, Seiya and the other Three Lights also attends the same school as Usagi. He sat behind Usagi in class and calls her 'odango-atama' like Mamoru. Naturally, Usagi is annoyed by his constant teasing but the more time they spend together, the more their friendship developed and they became close.

In episode 181, Seiya managed to ask Usagi to go out with him, without her knowing it was actually a date. They have a fun playful time together at the zoo, and they even hug each other accidentally. Usagi's heart is constantly pounding, and starts to realise Seiya might have something for her. Seiya advances towards her, and Usagi thinks too much into this. After realising he simply wanted to ask her to dance at the disco, she becomes slightly embarrassed. The lights go out due to a power outage and Seiya takes Usagi into his arm and tells her that everything would be alright.

Usagi: This warmth is like Mamo-chan, but it's different. It's a different kindness from Mamo-chan.

This outage however wasn't an accident, but rather caused by Sailor Iron Mouse, whose target was Kou Seiya. Amidst the battle, Seiya had transformed into Sailor Star Fighter without Usagi's knowledge. In this form, Seiya realises that Eternal Sailor Moon (not Usagi) knows Seiya! Things are starting to get complicated, though in the end Seiya convinces himself Usagi can't be Sailor Moon. He gives her a little bear keychain as a thank you gift for spending the day with him.

In episode 187, the girls from Usagi's school who formed the Seiya Fan Club weren't impressed by Seiya and Usagi's closeness. The leader of the fan club challenges Usagi and Seiya in a softball match, announcing that if their team loses, they must 'break-up'. Usagi denies the fact they're together, but it didn't seem to do any good. Usagi was pretty terrible at softball and Seiya patiently trains her to catch a ball all afternoon. Soon the night sky takes over and the two take a break while looking at the stars. He tells her he believes in her and that he doesn't want to lose the game. The truth was, Usagi didn't want to lose either and she tries her best which causes her team to win.

Things start to get bad in episode 188, when Aluminum Siren sets a trap for Usagi (after knowing her true identity as Sailor Moon). In order to protect her, Seiya transforms in front of her, revealing his secret identity. Usagi is shocked by the discovery but realises she has no other choice but to transform as well. Confusion, anger and sadness takes place, with the Sailor Starlights conflicting with the Senshi of the Solar System. Despite their differences, Seiya couldn't stop thinking about Usagi, he still believed in her. Usagi is also saddened because she doesn't want their friendship to end.

Seiya feels that she is very important to him, even more important than his mission to find his princess (Princess Kakyuu). This often creates quarrel among the Three lights, since he's the only one who trusted in Usagi, constantly defending her.

In a later episode, Seiya discovered Sailor Moon was vulnerable to Tin Nyanko's bazooka and he runs quickly to take the blast for her. He is injured severely, losing consciousness and starts to bleed. Sailor Moon is horrified and becomes desperate to help him. The other Starlights appear and take him away angrily, warning her to stay away from them. Their view on Sailor Moon turned into hatred as they blamed everything that happend to Seiya on Usagi. Their chances of seeing each was was slim. Haruka and Michiru also warn Usagi to break all ties with him.

Nevertheless, Seiya and Usagi try desperately to see each other. She wanted to apologise for his injury, and Seiya wants to tell her everything about his mission. He calls Usagi and invites her to one of his concerts, which was held in an Amusement Park. Usagi couldn't get a seat since the tickets were sold out, but notices a Ferris wheel and rides that. From such a high angle, she was able to see him on stage clearly. He was able to tell her his story through his song, all about Sailor Galaxia and his mission to search for his Princess. Soon they were able to reach a mutual understanding.

As time passed, Usagi felt more and more lonely without Mamoru's presence. We soon learn that Mamoru hadn't replied to a single letter she had written to him. After a battle Sailor Moon and Sailor Star Fighter have against Tin Nyanko, it starts to rain. On the school roof-top, Usagi starts bursting into tears and speaks from her heart. She misses Mamoru dearly, and she can't stand being alone anymore. These words hurt Seiya, and his anger shows... he was not angry with Usagi, but with himself. He kneels down and places his arms on her shoulders, looking into her eyes "Aren't I good enough?"

Usagi realises at this moment what she meant to Seiya....

Seiya and Usagi meet at the backstage before the Three Lights have their final concert. Seiya tells her that he is going to leave her after the concert. He confesses that he really likes her, and thinks about her all the time. Usagi doesn't know how to respond. He tells her it's okay if she doesn't love him in return. He leaned forward and gives her a kiss on the cheeks as tears stream down her face.

During the final battle, Sailor Star Fighter constantly put Sailor Moon's life above her own, she meant everything to her.

After the battle, the Sailor Starlights and Princess Kakyuu leave earth and return to their home planet. Seiya says farewell to Usagi and that he will never forget her. Usagi pretends to be oblivious about her feelings. He asks Mamoru to take care of Usagi and they depart. Usagi looks up to the stars and says farewell to Seiya...thanking her for everything.