Infact, Rei might be the one friend closest to Usagi. Usagi has many friends and they all get along really well. But what make Rei and Usagi really special is that they both enjoy getting on each other's nerves. And, there is a really noticible scene of what Rei meant to Usagi in BSSM Sailor Stars closer to the end of the story when the dying Sailor Mars is bids her farewall to Eternal Sailor Moon. Eternal Sailor Moon could very well talk to one of the sailor senshis, but she chose Sailor Mars. This might be a coincidence or maybe I'm making this scene mean a lot more than it should etc... anywhere's that's just my observation.

What makes Rei stands out is that she has the special ability read the fire. Rei likes to sit in front of a braze of fire and chants whatever that she chants. When the time comes, Rei would see images in the fire that are usually warning signs to prevent disasters from happening.

Rei's love interest in Threelights is Yaten. I don't recall anything specail happening between them, but I could be wrong. And, ofcourse, nothing could interest Yaten except his beloved princess Fireball.