Half-genies are the offspring of Human fathers and Genie mothers. 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.
Shantae, the guardian of Scuttle Town, is currently the only revealed half-genie in the game series so far, although Holly Lingerbean from Shantae: Half-Genie Hero is implied to have been a half-genie as well before being turned into a Memory.