Hotaru to Sailor Saturn

Hotaru becomes Sailor Saturn in the Sailor Moon S season, episode 125. She was completely taken over by Mistress 9 during the time. Hotaru was trying to regain control of her body, she couldn't let Pharaoh 90 take over the world.

She thought of Chibi-Usa, Usagi, and her father, Professor Tomoe. Those memories made her stronger, and Hotaru regained control of her body, thus defeating Mistress 9.

Sailor Moon couldn't become Super Sailor Moon, so, she wasn't strong enough to fight Pharaoh 90. Hotaru decided to use Death Reborn Revolution, and kill Pharaoh 90. The attack caused Sailor Saturn to be reborn as Hotaru Tomoe. She was taken by Michiru, Haruka, and Setsuna, and she lived with them until the Sailor Stars season.

In the Sailor Stars season, Hotaru grew rapidly, until one day Sailor Saturn, Hotaru's gaurdian, gave Hotaru her memories back. Hotaru was, again, able to transform into Sailor Saturn.