Whenever she visits, she leaves her wedding ring on the coffee table. There's always an excuse - it's heavy, it clinks against her cup, it needs cleaning. Kaoru doesn't mind. Whenever she visits, she slips on the ring he bought her instead, pale gold set with a translucent diamond. She never forgets to remove it before she leaves. Of course, Haruhi never forgets anything.
It's easier, now that her father in law has died: Tamaki leaves for business every other week, and Haruhi seeks comfort in Kaoru's arms instead before stepping out to defend the law come morning. She never sees the irony, in having such a strong moral compass and yet having an affair with another man. Kaoru doesn't bother to point it out to her; her eyes always go so sad when he reminds her of her husband, once his close friend from high school. So she leaves in the morning, her real wedding ring safely on her finger, and Kaoru allows the one she wears with him to await her return on the nightstand.
Nobody knows, not even Hikaru - Haruhi is used to keeping secrets, thanks to the Host Club. They have a routine, practically clockwork, and Kaoru always feels so warm when she's there that he forgets how it feels when she's gone.
But she's only ever back to Tamaki for a month or so before he jaunts off somewhere else, and then she's back in Kaoru's sitting room, her lipstick on his mouth, his hands in her hair. So meticulous; she uses a different brand of everything than she normally does when she's with him, in case anyone recognises the precise shade of red on his shirt, or the freesia notes of her favourite perfume lingering in his bedroom. Kaoru never bothers to do the same for her, because to everyone else he's just Kaoru, just a Hitachiin twin, and it doesn't matter if they smell his cologne on her. They've always been especially friendly, after all, him and Hikaru and Haruhi, and there's no reason to suspect anything when Haruhi is so happy with her husband, when the tanlines from her ring are so prominent, when Kaoru and Haruhi have never outwardly shown more than a fleeting, teasing affection to one another.
It always hurts, to watch her go. To watch her return to the life that she earned instead of the one she lives in secret, the life with the dashing husband and the high-flying job and the doting father. And her voice is always so measured, so careful when she leaves, and Kaoru wishes that she'd get angry at him or Tamaki or someone, just for once, but it's Haruhi, and she never does.
To her credit, she's always tactful, always careful from the moment she steps inside, from the moment she takes off her ring and allows him to kiss her until they both have to pull away to breathe. Kaoru is used to being second place, second choice, but she always indulges him, always pretends that this is the life that she lives constantly and not just in these stolen snatches of time that go by too fast, too fleeting, until Kaoru is counting down the days until her return. And she always keeps her face carefully blank when she visits with Tamaki, her caution forever paying off, Tamaki always teasing Kaoru about his secret lover, unknowingly laughing at his wife who runs back to Kaoru whenever she has the chance.
One day, it will come to a stop. Nothing lasts forever. But today he bites down on her collarbone and presses her into sheets that he'll have to wash tomorrow, lest anyone realises that it's Haruhi he's in love with, lest anyone realises that he's the other woman in this twisted love triangle. "I love you," he whispers, and she doesn't reply, she never does, but she's here for now, and it's close enough.