A limiter Rosario to Vampire franchise that suppresses the negative energy of Vampires making them less powerful and overall nicer.
What is a Rosario?
Currently, speaking the Rosario is a limiter which suppresses a yokai and one's own personality. It does this by creating a separate personality. There are still mysteries to unfold about the rosario, but some is already known about it. The rosario is not meant to be taken off by it's owner. Though, someone else that matches the rosario's aura can pull it off. The rosario is works as a Limiter.
Akasha Bloodriver is her mom. In chapter 33 of Rosario Vampire her mom attaches the rosario on to moka.
Moka treasures this rosario as a memento to from mother and is now in search for her. The rosario has been broken on a few occasions. In the anime, the rosario was used to save Yokai Academy's barrier separating the yokai world from the human world and Aono Tsukune had to get another one for inner Moka to see outer Moka again.