The Rosario Cross, also known as a Rosary or simply the Rosario, is a magical object worn by Moka Akashiya. It was bestowed upon her by Akasha Bloodriver, who is Moka's mother when she was still a child.
The Rosario Cross was designed to seal Moka's Inner personality and create an alternate personality, Outer Moka, based on the appearance and traits of her mother. When the Rosario is removed, Outer Moka disappears and Inner Moka awakens from her slumber.
The Rosario works as a medium for the two Mokas to communicate with each other. In the anime, the Cross used to power the school barrier is the same as Moka's, created by Issa Shuzen. In the manga, Akasha designed it to be only pulled off by someone that truly loves Moka and means her no harm, her "destined one", in this case that person is Tsukune Aono, as he had fallen in love with both Outer and Inner Moka.
The sealing powers of the Rosario can be temporarily canceled out by Lilth's Mirror and Belmont, but it can only be physically pulled off by Tsukune. Outer Moka can still remove it, albeit with life-threatening consequences and damage to the Rosario. When Outer Moka removes it herself after being captured, she nearly dies in the process. Headmaster Tenmei Mikogami repaired it afterwards, but nobody knew that the seal had been permanently damaged. Over time, the constant removal of the Rosario by Tsukune had weakened the Rosario's sealing powers, eventually reaching the stage where Inner Moka is unable to return to the seal. Fang Fang Huang and Ling Ling Huang disclose that the seal was never really fixed, the reason being that Mikogami had learned all his sealing techniques from another whose skill exceeds the Headmaster: Touhou Fuhai, head of the Wong Family and one of the Three Dark Lords.
The group travel to China, meeting Touhou, who agrees to fix the Rosario. By accident, Tsukune, Mizore Shirayuki and Touhou enter Inner Moka's sealed memories, revealing her childhood, her family, her mother's sacrifice and how she came to possess two personalities. When they return to the real world, Touhou quickly fixes the Rosario, surprising Akua Shuzen when Inner Moka unexpectedly reverts to Outer Moka.
Fairy Tale puts Moka in a special seal to break the one on the Rosario. However, Outer Moka willed the seal to persist as she did not want Alucard to awaken. Even when Outer Moka vanishes once more, Inner Moka doesn't release her full power, both to keep Alucard from awakening and to keep Outer Moka from vanishing.
It's revealed by Gyokuro Shuzen that the rosary is a unit that's linked to Alucard to control his body. This appeared to be false as Touhou revealed that the rosary is linked to Akasha sleeping inside Alucard. Later when Miyabi, who's revealed to be a clone of Alucard's human form appears and realizing that the rosary never had the power to control him, he frees his main self by destroying the seal completely. This eventually causes Outer Moka to die knowing that the rosario cross is what made her live.
When Tsukune places the destroyed rosario onto Alucard, it's returned to the rightful owner, Akasha who's recreated and revived from the rosario itself that's now attached to her taking control of her former king's body. Due to the rosario being placed on Akasha, Outer Moka's memories came back to Akasha since the outer was a clone of the leader of the dark lords. After the Three Dark Lords and Alucard vanished from the world, the rosario was left behind and everything in it was completely empty which had setted Inner Moka free knowing that she won't be sealed up by the rosario anymore. The rosario now remains as a memory of Akasha Bloodriver, however, Outer Moka's soul still lives inside Inner Moka thus the two Mokas becoming one person.
- Ironically, if you think about, Akasha actually chose who Moka would fall in love with. Inner Moka only fell in love with Tsukune because Outer Moka did first, and did everything she could ensure he surivived.