Updated design. Sorry for deleting yoru comments together with the last picture.

I based last design too much on a work of great artist Motchie, so chaining it a bit to avoid drama. Consider it as tribute

From Salario:

Halibelle, Gaia's human name, is a fun-loving partygirl, the only one of the Angels who enjoys having big tits and has no shame in flaunting her sexy body as she fights.

Sailor Gaia died saving Prince Endymion during the Silver Millenium by giving him her starseed. During the events of Angel Moon, Tuxedo Mask calls on the power of Gaia, and LCL figures out what happened to her. He then makes Mamoru a special collar that lets him transform into Gaia.
When Transformed, Gaia's memories and persona take over. Though since the collar was a rush job in making, it doesn't always work like it should.