The Character(s) that Mature Usagi
I feel like in every season of Sailor Moon there's that one character who matures Usagi.
In season 1 Usagi was an immature, whiny, little crybaby. She gradually got used to the job of Sailor Moon during the course of season 1, but what had matured her was the character Mamoru. After the whole "Moon Princess reveal, Phantom Crystal's finally appeared, past lives memories slowly coming back, & Queen Beryl kidnaps & brainwashes Mamoru" climax of season 1 Usagi realizes just how much she has loved Mamoru (underneath the surface of their bickering) all along & how much he truly means to her after he's been revealed to be Tuxedo Mask all along & has been kidnapped by Beryl. This is what matures Usagi most of all in this season, even in her civilian life.
In season 2 Chibiusa was that one character who had matured Usagi. I once read a comment online a really long time ago made by a teenage mom who was a fan of that season who had said that the relationship between Usagi & Chibiusa all throughout R had reminded her of her own relationship that she had with her own 4 year old daughter. She also said that taking on the responsibility of being a parent & taking care of a young, innocent life's well-being is the most biggest and important responsibility that any person can ever have (even moreso than saving the world in the case of Sailor Moon). That comment had stuck with me for years, and it really made me appreciate the Chibiusa & Usagi relationship for what it had done for Usagi's character in terms of growing up.
In season 3 Hotaru was that one character who had matured Usagi. Granted Hotaru was her daughter's best friend rather than her own, but still Usagi had treated Hotaru like she was her own daughter nonetheless. Hotaru had brought out the Jesus in Usagi, as Usagi was absolutely not willing at all, at any cost, to sacrifice the life of this poor, innocent, tragic young girl in exchange for the lives of others. Usagi doesn't believe in sacrificing good people's lives for the greater good. Her choice turned out to be the right one in the end, and even earned her the respect of Sailor Uranus & Sailor Neptune in the end too as well.
In seasons 4 & 5 however...I feel like that one character who matures Usagi (and also shows fans of the series just why it is that Usagi of all people is the leader & why we love her character Sailor Moon so much) doesn't exist?
In season 4 if anything I feel like Chibiusa gets that character for her. Chibiusa's character matures a lot through her relationship with Helios. Granted Chibiusa was already maturing last season through her friendship with Hotaru, but Usagi was still the main star & main character of the show. In season 4 it feels like Usagi's character takes a backseat & Chibiusa became the star of the show. Usagi felt like an important supporting character (Chibiusa couldn't transform without Usagi & Usagi needed Chibiusa in order to finish off the MoTD), but really it was Chibiusa's show.
In season 5 I don't remember any one character that helped mature Usagi's character even further?? Seiya perhaps? Chibi Chibi?? I dunno?