Although they fight and tussle a lot during the course of the story, Chibiusa and Usagi love each other very much. In fact, they love each other so much that it sometimes hurts my teeth. When trouble rolls onto the scene, Usagi can always count on Chibiusa and Mamoru to be there right beside her, supporting her with all their hearts. Plain clothesed Usagi often treats Chibiusa as a sister, and there is a sorority among Sailor Chibimoon and Sailor Moon as well, but ultimately, Usagi does begin to think of Chibiusa as her daughter and worry about her like a mother. Usagi is a little immature for motherhood through most of the series, but when the going gets tough, Usagi has a very mature yet innocent heart, and through this innocence she provides the comfort that the growing Chibiusa needs.
Chibiusa looks to Usagi as a role model. Although at first she can't understand why everyone looks up to Usagi, she soon learns the power of her heart and of her love and wants desperately to emulate her. Chibiusa wants to grow up and be as loving and giving (and as loved) as Usagi perhaps even more than she wants to grow up to be as fine a lady as Neo Queen Serenity. Usagi is brave and everyone can always depend on her when the going gets tough and Chibiusa admires that and is even a little jealous. She hasn't become the star of her own show yet, and playing second fiddle to Usagi constantly is a little tiring. Yet still, she knows that some day she will have responsibilities of her own and handles herself the best she can.
One of the biggest problems they lock horns over is their joint affection for Mamoru, but Usagi eventually comes to terms with this, even if she doesn't always remember to act like the grown-up she is. In fact, when Chibiusa develops an interest in Elios, Usagi almost misses fighting with her over Mamoru.
One of the most wonderful things about Chibiusa's constant trips to the past (and one of the reasons she's sent time and again) is that in the past Chibiusa can interact with her mother on a personal level that is impossible in the future. They can laugh and trade secrets and talk about the boys Chibiusa likes in a familiar way. Chibiusa can't do this with her future mother because she's so delicate and far removed. She seems like a fairy princess, not like a real person. Usagi is flesh and blood and Chibiusa has no problems harassing her and Usagi has no problems spanking her, if needs be. They love each other like a close family, while Chibiusa loves the Neo Queen like a shining star.
It is also interesting to note that of all of Usagi's friends and family, her soldiers, Tuxedo Kamen, and everyone else, the last person still standing and fighting with Sailor Moon at the Zero Star at the end of the manga is Sailor Chibimoon (along with Sailor Chibichibi, who is a manifestation of Sailor Moon herself). When Galaxia throws Tuxedo Kamen into the Galaxy Cauldron, Chibimoon fades from existence and Sailor Moon is devestated, having lost the last little bit of everything.