Maison Ikkoku is a romantic comedy, meaning that, through sometimes funny, sometimes emotional situations, the author makes us witnesses of the character's evolution.
But Maison Ikkoku is a little more than that, since it's one of the few adult story of Rumiko Takahashi. None of the characters has ESP, is immortal or comes from another dimension/planet...
Maison Ikkoku's heroes, as opposed to many Japanese animated series and manga, are adults. The main characters are in their twenties and live independently from their family. What a change, after so many high-school-type brats, 14 years old, just starting to learn men and women are different ! ^_^
It's also for that reason that Maison Ikkoku's manga has been published in a adult publication and is a Seinen manga as opposed to shonen manga. However, it's characters are, in many ways, as young of body as of spirit...
Kyoko is not a "frozen" character in her feelings or her attitude, it's a character Rumiko Takahashi make change and live...
When Kyoko is first introduced, she's near 20 years old. It's young for working as a kanrinin in an mad house like Ikkoku-kan !
Later, we will learn that she's a widow. Twenty years old, it's also very young to be married : actually, she married at 19, in fall, just after graduating from high-school... and she became a widow six month later, in Spring. In October of the same year, she decides to spread her wings and makes her entrance at Ikkoku-kan.
In the time before that, she wears short hair, like when she was a high-school student. On the pictures and in her memories, she always has cut short and square hair. In my opinion, that haircut is a symbol of her youth at that time. It was a very young girl, just grown from adolescence.
When she makes her entrance at Maison Ikkoku, she's long haired and looks more like woman already. But she keeps on wearing jeans and sweatshirts, at least in the beginning.
Later in the story, Kyoko will dress in a more feminine and a more "mature" way, wearing long skirts and dresses, more classic.
In the end of the story, Kyoko will be around 27 years old ! Our story then spans over 7 long years, during which as much her as Yusaku will have time to mature and to learn the lessons of life.
Because of her early marriage, Kyoko never studied beyond high-school. She stopped any study for marrying Souichiro. She will admit herself, while walking among the students in Yusaku's university, that she misses all that life and that energy. Hints about some regret for what could have been if she hadn't married ? Nevertheless, a whole part of her life has been taken away by her marriage and her widowhood...
Even if Kyoko is a responsible woman for her age, that doesn't keep her from being immature sometimes. She's an adult, yes, but she's short tempered, she's jealous, over-reacts and sometimes (often) acts like a child : she's whimsical, sulks, gets angry, but she's also filled with wonder in front of an Xmas tree, laughing. Ikkoku-kan's tenants, immature themselves, allowed Kyoko to keep that energy and to stay that way : young.