Cortana is an AI cloned from the mind of the genius Dr. Halsey, and was instrumental in helping Spartan John-117 save humanity from the alien Covenant. Her form is holographic, allowing her to tailor her appearance. She chooses to appear as a small, curvaceous blue woman.
Janet van Dyne was the original Wasp, wearing a suit developed by Hank Pym that grants her the power to shrink and grow by manipulating sub-atomic Pym particles. She has also undergone genetic modification that allows her to grow a pair of gossamer wings while tiny.
Salu Digby, otherwise known as Shrinking Violet, is an Imskian. Along with the rest of her race, she has a natural ability to shrink to microscopic size and retain most of her physical strength.