Rini is the future child of Serena and Darien and also the future Sailor Moon. Rini, despite many other profile claims, doesn't want to cause trouble with Serena, all she really wants to do is to help her mother and father. Rini is very delicate when it comes to her friends, she chooses them carefully and treats them like gold. She is very dependable when it comes to personal matters.
Rini enters the "Sailor Moon" story in the 4th book. She demonstrates her power by traveling through time to the past. When Rini arrives in the past, our present, she is an emotional train wreck. She is sad and closed off from every one except Darien. She's not in love with him, but in desperation for a parent. She doesn't even talk to Serena. I think she doesn't want to hurt her like she did her mother. When Rini picked up the Silver Crystal a power flux happened in the Crystal Castle (which is made completely out of the Silver Imperium Crystal). At that time the Black Moon felt the change and started to attack. Neo-Queen Serenity(the future Serena) ran out side of the castle to find Rini and was hit by the attack. Crystal engulfed her as she started to fall and then was moved into the center of the Castle. Darien, Amy, Rei, Lita, Mina, Luna, and Atremis all were hit and fell into a deep sleep, their bodies preserved, and the true destruction began. Everyone started to fall asleep, then die, and finally disappear. When there was no one left, and the Black Moon was coming for her, Rini stole one of Pluto's keys and traveled to the past. All Rini wants to do is to save the people she loved in the future. Her noble act would almost tear her parents apart, destroy the past, and kill Jupiter, Mercury, and Mars.
When Rini comes into the picture, Serena and Darien, along with the others, were just getting back to normal from the Negaverse battle. Rini falls out of the sky and lands on Serena just as she was kissing Darien (good timing!). Rini just wants to get Sailor Moon to come to the future to help her save her parents. Rini also just wants to have some friends and attempts to do so when she goes to the past. She unintentionally puts Serena and Darien into a tight spot. Rini begins to doubt herself when she returns to the 30th century.
Wiseman feels Rini’s doubt and plays on it, giving the child enough power to awaken as Black Lady. She even has enough power to brain wash Darien and get him to love her. It takes the death of Pluto to snap all the Negative energy out of her and to finally become a Sailor Scout. In becoming a Scout, she triggers a series of events that awaken the world.
As a Scout Rini is dependant on Sailor Moon to attack with her. She is a very weak Scout till her Amazon Court join her during the StarS. Even then she is dependant on them to attack with her. Because the story ends when it does, we never know what kind of scout Sailor Chibi Moon becomes, but there are signs that she becomes a very strong scout.
Rini’s attacks have to do with Sailor Moon or some kind of “child-like” attack. She uses a weaker version of all of Sailor Moon’s attacks until she becomes Eternal Sailor Chibi Moon and her court awakens in the 30th century (see StarS book 2 & 3). But even then her attacks are still too weak and she relies on the Amazon Quartet to aid her.