A constant of Kyoko's behavior, throughout the series, is her jealousy toward those who have declared having feelings for her. She, who swore never to love a man again, is very attached to sincerity.
Shun Mitaka, the tennis coach (whose name means "Three eagles" and who is also in love with Kyoko), and more particularly Yusaku, will endure it for years.
At the slightest hint, the slightest misunderstanding, she gets upset and becomes cold as ice, meaning for the culprit that he better finds a good explanation. Being so, she's so feminine and so adorable, we can only feel drawn to forgive her.
One must say that Rumiko Takahashi, whose pronounced taste for light comedy's misunderstandings is well known, has planned for our protagonists several situations, putting a hard strain on their mutual confidence.
Mitaka, always surrounded by pretty young girls, ready to fall for him anytime at the tennis club, will get scowls and disapproving backs. Or again, when glimpsed in the arms of a woman (just running away from a dog he's allergic to) he will be ignored for weeks. The only serious rival for Kyoko will be Asuna Kujou ("Nine passages" ^_^), a young girl from a wealthy family (talking very softly and a dog's maniac), who has been introduced to him by his uncle, who plots to join their families. Kyoko will not be really jealous of her and she won't really fight for keeping Mitaka. And that revelation, dawning on him, will make him understand that she never really loved him and that's only the pressure from her family and his pushy behavior that made her consider, a moment, that he could be an eventual husband.
Yusaku, will add stupidities and misunderstandings : we could almost believe that every girl he meets gets interested in him. But he only wants one : Kyoko. He will even get an "official girlfriend", Kozue Nanao (whose name means "Seven tails"), but strangely, Kyoko will never really be angry at her... but Kozue is adorable and wouldn't hurt anyone.
When I write "girlfriend", you must think the Japanese way : it's a person you go to see movies with, to who you offer gifts at Xmas, that you try to see often (although Yusaku doesn't seem very eager, except when he's hungry and can get a free meal at her parents) and that you, sometimes, dare to hold by hand or the shoulders, but not more. Kyoko never considered her a serious rival (in what she was wrong) until the very end, but a misunderstanding having led her to believe Yusaku had made a proposal to Kozue, will get her to go straight to her parents home, refusing to see him for days.
Other rival, and much tougher that one, is Ibuki Yagami ("Eight gods"...). She's one of Yusaku's student when he's a substitute teacher (he's majoring in education) at her high-school. 16 years old and ready to do anything to sneak in her Godai-sensei's life, she will be the one who will challenge Kyoko the most and the one by which she will be the more questioned. Ibuki is not blind, like Kozue, nor stupid like Nikaido, she suspects that Kyoko has feelings for Yusaku and she will attempt to provoke her in order to make her come out and settle things once and for all.
Many times, Yusaku, in the midst of a jealousy storm, will wonder if that attitude doesn't show how much Kyoko cares for him... but he's never sure of anything and Kyoko's confusing signals have a lot to do with it.
Of course, she doesn't acknowledge having feelings for him, even if she sometimes wonders about her own motivations. She keeps Yusaku under a close watch and yet : "he's only a tenant like the others !" she says...
All the story arcs involving Ibuki, anyway, will probably help Kyoko to accept her feelings and to change by leading her to question herself about the reasons of her jealousy and about her fear to love again.
Back on the subject of jealousy, a crisis in connection with a sweater offered by Kozue to Yusaku, will, indirectly, send him to hospital a leg broken in a bad fall.
Actually, Kyoko, never stops to wonder about Yusaku's commitment with Kozue and that ends up being obvious. He says that there's nothing between them, that she's just a friend, but neither one of the two girls in this triangle shares that point view, of course...^_^
In the series, that over-possessiveness is the best hint about Kyoko's feelings toward Yusaku. The author unfolds gradually Kyoko's thirst for absolute feelings and her refusal of compromises : if Yusaku declared he loves her, he must be faithful to his words... And if herself, decided she loved Souichiro forever, she would consider it had been a lie all along, were she to start loving another man.
However, very quickly, she gets involved in Yusaku's life and she gradually discovers that, seemingly a late teen-ager, he's got a big heart and the will and perseverance to carry out his dreams.
So, among all these girls, which one is the most serious rival for Kyoko ? Without any doubt, I'd say it's Kozue. Young, cute, ingenuous and convinced that Yusaku loves her too, she wouldn't like anything as much as to marry him. And Yusaku's heart wavers, between a Kyoko seemingly unattainable and a happy Kozue already conquered. One of Yusaku's great merits is to have been able to remain faithful to his first feelings, which, if they were only superficial in the beginning, have only deepened with the passing of time and years.
Ibuki is never a menace to Kyoko, Yusaku seeing her more as a burden than a conquest, she will be more of a trigger in the dawning realization of Kyoko : if she's jealous, it's because she has very strong feeling for Yusaku.
The last jealousy crisis of Kyoko will be the one following the story arc with Akemi and the love-hotel. Yusaku having been seen in the entrance of one of these special hotels, whose rooms are rented for an hour, holding Akemi by the arm (actually, he was only coming to pay the room because Akemi was too drunk to do anything), Kyoko will even get to slap him. That one puts an end to a long series triggered by several female characters, hovering around Yusaku. Tough to endure a jealousy crisis without even be sure to be loved, isn't it ? ^_^