"Serena, there’s an old proverb that goes like this 'You shall reap what you sow.'"
Back in the Silver Millennium, Luna was an advisor to Queen Serenity as well as a servant to Princess Serena. She was present when the Moon Kingdom fell, and was one of two cats (the other being Artemis) that sounded the alarm that the moon was under attack. She stayed by Queen Serenity's side when the queen used up her own energy to use the Imperium Silver Crystal (hereafter referred to as simply "the Silver Crystal") to seal away the Negaverse and send her daughter and her court to the future. Luna along with Artemis was put into a long sleep to be awoken when the Negaverse attempted to get free of its prison. It was her duty to awaken the Sailor Scouts and find the moon princess though her specific memories about the princess were suppressed.
She found Sailor Moon, a crybaby by the name of "Serena Tsukino", first, and they were off to a rocky start. Luna was focused on her mission and frustrated that this girl didn't seem to really want anything to do with her destiny. Regardless, the guardian cat provided advice-usually unwanted-to the girl in an attempt to guide her. Upon finding Sailor Mercury (Amy Mizuno) and Sailor Mars (Raye Hino), she had help in keeping Serena on track…though in some ways it made her job more difficult due to bickering, talks about boys, and other distractions. Of course, having three Sailor Scouts made it so that she didn't have to be right in the thick of the battlefield all the time though she still provided assistance in what ways she could.
She also found Sailor Jupiter (Lita Kino) as the search for the Rainbow Crystals that would form the Silver Crystal began. It ended up being a fight between the Scouts, the Negaverse, and the Scouts' own ally Tuxedo Mask over who could get all seven Rainbow Crystals first. Near the end of this hunt, they were joined by Sailor Venus (Mina Aino) along with Artemis whom Luna remembered, even commenting that the other cat hadn't changed.
Not too long after all of the Scouts were united, there was a showdown atop the Starlight Tower between Zoicite and Tuxedo Mask (Darien, a young man that constantly picked on Serena). It was there that the Rainbow Crystals came together in response to Sailor Moon's tear when Tuxedo Mask was badly injured, forming the Silver Crystal. This revealed that Serena was the moon princess all along. Following that revelation, Luna and Artemis revealed some of the Scouts' past on the moon that they themselves were only just beginning to remember.
Though the Sailor Scouts managed to get away from Malachite with the Silver Crystal, Tuxedo Mask was captured and taken back to the Negaverse. It was as they were returning home to regroup that Luna awoke in Nautilus.
Luna is a rather prim and proper lady despite being in the body of a cat. She tends to speak rather formally to others and is generally polite and respectful. She's a rather focused individual, taking her duties as guardian and advisor rather seriously which can frustrate her when her charges don't take their own duties the same. This leads her to coming off as a nag.
Of course, she only speaks when around someone who knows what she really is. To those outside of the Scouts or who haven't discovered the secret for themselves, she behaves much like a normal cat. However, she doesn't simply drop the feline behavior when she's with someone who knows what she is. She's quite intelligent but she is still a cat. For example, she doesn't see a good scratch behind the ears or under the chin as demeaning; she rather enjoys such gestures.
Luna also has a teasing sense of humor. She'll make fun of Serena on occasion, and Artemis is another favored target of hers. At times this teasing tends to wander into "somewhat cruel" territory such as when she drew a picture of a fat Sailor Moon to poke fun at Serena's obsession with her weight.
Despite the nagging and teasing, Luna does honestly care about others. She can take on a rather maternal attitude when something is truly bothering someone she cares for. She doesn't always have all the answers, but she knows how to sit and listen at the very least.
When she gets frustrated enough, Luna can display quite a temper. She'll shout and hiss at the person, even threatening with her claws if it will get her point across.
Even though most enemies are many times her size, she's quite brave and will do her best to fight using only her claws and teeth if she must fight. This is usually only effective as a distraction rather than causing much damage to an opponent. She does have a tendency to attack the eyes and face in order to make the opponent more vulnerable, but this is usually met with mixed success.
Since Luna is in the body of a cat, she possesses all the basic feline abilities: agility, superior balance, night vision, acute hearing, and retractable claws. She also has the ability to keep up with Serena and the other Scouts when they're running away from either danger or an awkward situation in which someone might've overheard Sailor business.
Her most notable power is the ability to talk though she doesn't do it very often in front of those outside of the Sailor Scouts. When she speaks, it's with a British-sounding accent.
She can sense evil in the area and can even sense a Sailor Scout though she doesn't always recognize the latter feeling for what it is. For example, she originally mistook Amy for an agent of the Negaverse though she apologized afterwards. She generally describes this sense as either a bad feeling or simply strange vibrations.
She's also quite intelligent, even able to use a computer. This is useful when giving advice to the Scouts, particularly in battle. Unfortunately, her understanding of relationships can be a bit lacking, particularly romantic relationships.
She has the ability to conjure items for the Scouts seemingly out of thin air, usually performing a back flip as she does so. This is how she gave Serena her locket and Amy, Raye, and Lita their transformation pens along with Amy's mini supercomputer.
She can also reawaken suppressed memories in the Sailor Scouts, creating a connection using the crescent moon on her head. However, this ability has only been used twice: once to reawaken Serena's memories as Sailor Moon and the moon princess; the other time to reawaken the remaining Scouts all at the same time. (This was necessary since after the defeat of Queen Beryl, their memories were suppressed but they were still needed when a new threat had come to Earth.) It's unknown whether or not this ability can be used on anyone else.
Italics indicate cast mate.
Dana was the person who found Luna when she first arrived in Nautilus. Though Luna has yet to reveal that she can speak to Dana, she likes the girl and is grateful for the help she gave.
Raye was the third Sailor Scout that Luna awakened. Sometimes Luna gets frustrated with how she and Serena will get into fights over everything from comic books to boys, but she knows the pair are good friends if not best friends.
Luna is no fan of Rex after getting off to a bad start. She mistook him for an intruder (and a dangerous one at that) in the temple and about clawed his face off in order to chase him out.
Serena was the first Sailor Scout that Luna awakened. Luna's relationship with Serena can be a bit strained at times due to Luna's focus on their mission while Serena wants to have fun. However, she has watched over Serena even back in the Silver Millennium.