Hotaru Tomoe is the drama of the Senshi. Her life has been nothing but suffering since she was little. Hotaru, when we first meet her, is very dark and cryptic, not friendly, and shut out of the world. But when she's reborn, that all changes along with her mission.
When Hotaru enters "Sailor Moon", she's half robot, half human. A lab accident when she was little killed her mother and mamed her body. Her father's mechanical and biological genius helped her live. When Rini befriends her, Hotaru starts to come out of her "shell", but also starts to become Mistress Nine. Even after she's reborn and grows again, Hotaru still is attached to Rini. Out of the four Outer Senshi, Hotaru is the closest due to the fact that her and Rini are best friends.
Hotaru becomes Mistress Nine when she swallows Rini's Silver Imperium Crystal and Rini's soul. There is a miscoception that Mistress Nine and Hotaru are the same person: that is slightly true, they share the same body but not the same soul. Hotaru became locked in her own mind until her Planet Power started to kick in and she killed herself along with Mistress Nine.
The Outer Sensh reappear in SuperS 3. We get to see Hotaru as a small child, happy, playful, smart..... destructive. We also see that she still has her healing power when she breaks Michiru's Royal Copenhagen dishes and cuts herself up. In SuperS and in StarS, Hotaru has shed her past and become a bright cheerful little girl. She even attends school with Rini. We can tell then that Hotaru has become the nice girl we want her to be.
Hotaru doesn't "awaken" as a scout, but rather Resurected as her scout form. This happens when Sailor Moon hurdles herself into Pharoh 90 inorder to destroy them. Sailor Saturn's weapon is the Silence Glaive. It's the most powerful weapon next to the Silver Imperium Crystal. Saturn's mission when she is awakened is to destroy the world, she almost does, but Princess Serenity resurects the world and kills Pharoh 90 in the process.
When Hotaru reverts back into infant form, the outer senshi haul her off so they can raise her and control her powers. Hotaru becomes Saturn again when Serena and Darien fall ill and the premenition of death comes. She changes her mission and now becomes a protector of life.