Haruka is the proprietor of the Cafe Hinata and was dorm mother of Hinata House until Keitaro arrived. For a majority of the manga she has only a small role. She is usually the voice of common sense and offers advice to the more high-strung characters when needed. Haruka offers Mutsumi a place to stay when her house burns down, which leads to Mutsumi also working at the cafe.
Haruka and Seta
Most of Haruka's and Seta's past is revealed in vol.9. Seta comes back to Hinata House after traveling abroad, and he asks to see Haruka. Keitaro and Naru lead him to the cafe, where he surprises them by offering Haruka a ring. To confuse matters further, Haruka turns Seta down flat and throws the ring back at him.
Naru and Keitaro each talk to a member of the older couple and discover that Haruka and Seta used to be a couple back in college. They became a pair of globe-trotting adventurers until one day Seta confessed his feelings to her. Haruka never game him a reply, and with their exciting lifestyle one thing led to another and they just lost track of time. Seta didn't help matters by moving in with another woman.
When the gang travels to the beach, more of their past comes out. Sarah(Seta's daughter) explains that her mother used to go adventuring with Haruka and Seta. Seta had feelings for both of them and was unable to make a decision. He promised that by the time he was thirty he would make his choice, but Sarah's mom died before this happened. According to Sarah, Haruka basically won by default which is why she holds a grudge.
That night, Keitaro and Naru eavesdrop on the older couple and it appears that their relationship is beginning to thaw. Haruka tearfully tells Seta that they're over and shouldn't try to be young like Keitaro and Naru. Then a smile passes across her face and she tells him that since the day made her reminisce about the past, she would wear his ring... but only for the night.
Before vol. 13, Haruka had always been sort of a homebody; staying home whenever the gang would go off traveling and having adventures. When Seta shanghais Keitaro into coming with him to the island of Molmol, Haruka surprises Naru by going after them(though according to Haruka she's just going to beat Seta up). Sure enough, the first thing she does upon seeing Seta is to punch him. She later has a taste of her old life while hiding out from Su's forces in an attempt to stage Keitaro's rescue. Most importantly, she and Seta make it to the Todai Ruins. Seta again asks her to marry him, and this time she agrees. Wearing a dress that Mutsumi had brought from the capital city, she and Seta are wed. After the wedding, Haruka and Seta decide to stay on the island and complete the excavations.
Haruka, Seta, and Sarah return to Hinata House for the wedding that takes place.