Half-genies are the offspring of Genie mothers and human fathers. They are all female and do not possess the full powers of their Genie lineage. In spite of their lack of skills, the daughters of the Genies made a decision to pool their powers and stand against evil as best they could. Few and far between, these half-genies traveled the land, each choosing a kingdom to guard until peace could be won again. As they struggle to keep the peace, new villains emerge and ancient foes return to wreak havoc on Sequin Land.
It's mentioned in the first game by an NPC that half-genies are often born with unusual powers, such as one being able to shoot milk out of her eye. Shantae's power is magic bellydances which help her achieve various feats, including transformation into various creatures or teleportation. She also can use her hair as a weapon, but it is revealed at the end of Shantae: Risky's Revenge, and shown in Shantae and the Pirate's Curse, that it is likely unrelated to her Genie lineage.
It is revealed that Shantae is actually not the only half-genie. According to Harmony, the oldest of the Half-genies, there is only one generation of half-genies as they have not been around for that long. She further explains to Shantae that their kind will eventually go extinct if the Genies do not return.