It was jealously, mostly. At least at the start.
It was just after the Shaman Fight, when no one knew what to do with themselves except to stick together like that had for the last year. They were celebrating, but it was a sober celebration, because Hao had won.
They all thought the world had gone mad when Anna suddenly got sick. After the explanation, they thought it even more so. Anna was pregnant.
Once again Ren felt that feeling he always got around Yoh. Insecurity. Yoh was pulling ahead of them, growing up faster and doing things none of them had ever done. he was hardly 15 and he was going to be a father?
Ren felt a need to catch up, to be his rival even in this. So he grabbed the first girl he found. That girl was the Iron Maiden Jeanne.
No, he didn't take her because she was first. He had reasons. Ren wouldn't have just anyone, either for competitiveness or pride or the high standards with which he was raised.
Jeanne was strong, and for Ren, that was important. She may have been equal or greater than him in strength. She had saved his life when he had shown weakness. He could respect her. But he was drawn to her for a more specific reason. He was an atoner, a sinner; he wanted to be forgiven, and she was a goddess.
Both of them knew she wasn't really a goddess. She had no more connections with God than any other Shaman. But Ren had at first seen her as everyone saw her, as the X-Laws tried to hard to portray her; he wanted to believe that there really was something holy about her, that he could forgive his sins.
She couldn't of course; she had sinned as much as he had. She too had killed many in pursuit of false morals and wanton hubris. Perhaps she was searching for forgiveness too, maybe that's why she let go of her of her goddess-facade and wanted to be a normal girl, in a normal relationship. Maybe that's why he accepted Ren.
He knew he was lucky to have her. He knew she was beautiful; her brilliance pierced through even his dark heart and left him in awe. Those were the times when he was sure that he loved her. There were other times, however, when he wasn't.
There were times when he regretted the whole thing. More than anything, he had wanted his child to have a happy childhood. But he didn't know what that was like, and neither did Jeanne; she had been locked in a torture device before she was 13. The only thing they could do is try to learn from the wrongs of their own past.
But the regret also came from his own uncertainty. He felt like he had taken her under false pretenses, decided things too fast in his rush to meet Yoh.
His was a heart always drenched in a degree of fear. The losses he'd felt during the Shaman Fight had left him uncertain about many things, the pride he'd felt in the beginning long gone. Ren was also young, unreasonably so to be looking for love, for marriage, for children. But when fear of uncertainty deterred hum, the greater fear of failure, of falling behind, drew him forward.