The spirit of the necklace is a character in Princess Maker 2.
If Olive has a set of Venus Jewels, every year on her birthday they will summon a spirit who grants Olive the blessings of Venus, the goddess of beauty. This increases Olive's Refinement, Charisma, and Sensitivity by 1 point for every year old she is (11 on her 11th birthday, 12 on her 12th, etc).
Keep in mind that if Olive ran away on the month of her birthday, the spirit will not bless her. So it's extra important to keep her from doing so.
Buying multiple venus jewels will not make her bless Olive multiple times, though the jewels themselves will still raise Charisma and Refinement.