Usagi’s relationship with Chibi Chibi is one of my favorite elements of Stars, if not the whole series.
It’s a very refreshing change from her relationship with Chibiusa. Although Chibiusa and Usagi have a very strong, loving relationship, it’s still fraught with pretty jealousy and silly squabbles. Although they always work it out in the end, we only rarely see Usagi step up into the mature role, the protective role.
Then along comes Chibi Chibi.
Chibi Chibi’s first appearance is very similar to Chibiusa’s in a few ways. They’re both pink-haired little girls who quite literally fall from the sky and into Usagi’s life. They both brainwash Usagi’s family into accepting them and seem to have ulterior motives. But there are some notable differences. Usagi’s first meeting with Chibi Chibi is noticeably less hostile than her meeting with Chibiusa. Most notably, Chibi Chibi doesn’t pull a gun on her. :P
But aside from that, while Chibiusa lands on Usagi, steals a kiss from her boyfriend, and pulls a (fake) gun on her and threatens her, Chibi Chibi assumes the role of a lost child. They are introduced when Usagi catches the parasol Chibi Chibi was holding, an act that quickly endears the child to her, which leads Chibi Chibi to start following her around (when you rewatch this episode after finishing the series, it’s clear that Chibi Chibi was able to sense that Usagi is the strong, kind warrior she’s been sent to look for). Throughout the whole experience, Usagi is never angered, just confused and a bit concerned.
My point is that I feel as though their first meeting set the course of their future relationship. Usagi got off to a very rocky start with Chibiusa; her start with Chibi Chibi, while strange, was mostly okay.
I think the ages of Chibi Chibi and Chibiusa (as well as their personalities) also play a huge role. Chibiusa, while clearly a child, is very mature and precocious. Although she can be rather bratty and childish at times, for the most part she acts much older than she appears. Chibi Chibi, on the other hand, is a very sweet, very cute little girl; she’s practically still a baby. She can barely talk (only saying “chibi chibi” and repeating the end of sentences) and clearly still needs someone to take care of her.
I don’t feel like Usagi ever felt much of a need to be protective towards Chibiusa (except for in dangerous situations, that is… in that case, WATCH OUT if you’re the one threatening her baby). Chibiusa is pretty mature and has shown that she can oftentimes handle herself. On the other hand, when Usagi looks at Chibi Chibi, she sees a very young child who needs constant protection and love. Instead of her sisterly relationship with Chibiusa, Usagi basically becomes Chibi Chibi’s surrogate mother; protecting, loving, and caring for her.
As I mentioned above, I feel as though this is a very nice change and further serves to distinguish that Chibiusa and Chibi Chibi are very distinct and separate characters (which is why it seriously peeves me when people say that Chibi Chibi is just a replacement Chibiusa). Through her relationship with Chibi Chibi, we get to see Usagi’s maternal and mature side.