Luna serves as Usagi's somewhat chief adviser. With her guidance, she was able to inform Usagi to call on a transformation. She's smart, too! Enough to gather all the Inner senshi together in order to fight the Dark Kingdom.

And sometimes find herself in tough situations!

But Luna helps a lot in every way she can!
She even attacks the villains, herself.

Luna cares very much about Usagi and all the senshi. She can be assigned to watch over Chibiusa, sometimes!

And let us not forget what Luna feels towards Artemis.

And other people besides....

In the Bishoujo Senshi S movie, Luna falls inlove with a human being, Kakeru. The Silver Crystal grants Luna to be human and she is so pretty in her human form! However, inspite of their kiss, Kakeru is destined to be with someone else, Himeko. And Luna finds comfort in Artemis (whose human form in the manga is handsome!).

And her moment with the Fire Senshi? In both anime and manga, she thought that Rei could be the princess with her looks and character. She found out later who the princess was, of course, and continued to play an important role in the whole series.