Rei has had three major romances throughout the series. The first is found only in the manga, the second is with Darien, and the third is Chad.
At the Moon Kingdom, Prince Darien had four protectors. They were Jadeite, Nephrite, Zoesite, and Kunzite. Princess Rei and Jadeite were in love, as were Nephrite and Lita, Zoesite and Amy, and Kunzite and Mina. This was all over when Queen Beryl arrived and brainwashed the four former protectors. They became her four generals. When Rei was revealed as Sailor Mars in the present, she killed Jadeite.
Before I say anything else, this was before Bunny was revealed to be the Moon Princess! Anyway, one day Rei ran into Darien (litteraly). She convinced him to take her out on a date. Little did they know, Bunny found out about it. She took Melvin and followed Rei and Darien around for the entire date. After Bunny was revealed as the Moon Princess and Darien as Prince Darien, Rei ended the relationship saying they were meant to be long before Darien and she.
One day, a boy showed up at Rei's family's Temple. He had long shaggy hair, and didn't shave as often as he should. He fell in love with Rei at first sight, and decided to stay at the Temple. His name was Chad and said he had been a "rock star." It turned out his parents were very wealthy and could owned a ski cabin where an episode toward the end of the first series takes place. Anyway, he has saved Rei a few times, and even fought Haruka once for her. (This was a misunderstanding and you find out about it here.)