Allison Takahashi is a 16 (technically 97) year old daughter of Kisho Takahasi, the 2000 year old Akane clan leader. Akane are superhuman immortals that are very similar to vampires. They have sharp, intense eyes, light tan skin, and are inhumanly beautiful. Akane have two states of being: immortal, and human. After exhausting themselves, such as by performing long distance teleportation, they take on their human form, but can then change back after a few hours. When they change, they can alter the age they appear to be.
Akane have about the same speed and strength as vampires. They can also teleport and some have hypnotic eyes (useful for erasing the memories of witnesses). Certain ones also have superpowers. Ally can unleash waves of sonic from her forearms, and also has enhanced fighting skills.
Akane are created when an existing Akane's genetic information is fused with that of an existing human. Kisho Takahasi made Allison an Akane after her parents died in an accident. She was created to be the future clan leader. For example, normal clan members can teleport a few miles, but Tanya can go anywhere on the globe.
There are about 300 Akane living in the world, and the organization has about 50 members. Their job is a lot like that of the Volturi. Organization members monitor Akane living among humans and assist if things get out of hand (like if someone destroys an early 20th century schoolhouse cough Ally).
They survive by eating human food, as often as they want. It tastes very good and strengthens them, but is not absolutely necessary. They can also eat things like rocks, dirt, and blood.
Since human blood is not Akane's primary source of food, they don't look down upon them quite as much as vampires do. But they still feel superior.
(Ally is pretending to be 16 right now because that's how old Edward is supposed to be in Forks at that time.)