The inner senshis' primary reason is to protect the Princess, who is actually Usagi.  They were close to each other since they also have to work closer.  Know more about your favorite senshi and click on to the links provided for you to be directed to a more detailed and comprehensive site related to her.

TSUKINO USAGI/Sailormoon She is otherwise known as Princess Serenity/Neo-Queen Serenity.  Although Usagi can be 'childish', she is really quite caring and protective of her friends.  She would do anything to protect them.  She and Minako share a special bond since both have many similarities. Leader and the Princess MIZUNO AMI/Sailormercury Intelligent, caring and especially understanding, Ami is a quiet and feminine person and often has a kind word to say and a thoughtful deed to do to make you happy.  She and Minako-chan aren't that close and are complete opposites but Minako-chan can be sure that whenever there's an exam, there's Ami-chan! Ice Senshi's Lake of Illusions HINO REI/Sailormars Hino Rei is quite close to Minako-chan in the manga, you can even say that they are bestfriends.  Rei-chan has a fiery personality and expressive eyes which you can be sure would make a man's heart melt.  She's the only one who studies in T*A all-girls school and she's also a hinto priestess who lives with her ojii-chan. Fire Senshi's Realm KINO MAKOTO/Sailorjupiter Feminine, but yet, also tomboyish, I would say it was a good combination and our proof is Kino Makoto!  Mako-chan is bestfriends with Minako-chan in the anime.  She's very mature and knows a lot about love, and so, the senshi asks for her help when they have a problem.  Even Chibi-Usa ran to her when she was troubled. Dark Echo Grounds of Thunder Senshi HOME | AINO SENSHI | OUTER SENSHI | GUARDIAN CATS | IN RELATION