Kuwabara wanted to walk in the sun with Keiko. He wanted her to fix his collar where everyone could see, wanted to help her into her coat and carry her bag for her. He wanted to carry her bag, was how deep in he was. He wanted to kiss her without hearing her apologize after - not for the kiss, but for hiding him away to do it.
Keiko wanted those things too. Keiko liked the way he rolled up the sleeves of his t-shirts and the waist of his jeans. She liked smoothing his shirt back into place after they kissed and the way he slid his hands out from under the back of her jacket. It was solid and sweet and real in a way she'd tried to be with Yusuke for years and just...couldn't. She was happy. Kuwabara made her happy.
But she still didn't want to hurt Yusuke. At the very least, she wanted the ability to explain it to him herself, instead of him finding out about it third-hand. She knew if anybody saw them, Yusuke would hear about it the moment he got back.
So they hid, and Keiko's shirts stayed firmly on her shoulders. Most of the time.