Seventeen-year-old Cassie is a shy, straight-A student with low self-esteem thanks to years of bullying and tragic events in her life. She is a bookworm, and expects to head off to college and get a great (if not boring) career, until she meets Alex Grayson, and her whole future flips on its head. 
She lives with her mother, Marie, and her two best friends are Gina O'Halloran and Izzy Greenslade.

Alex appears to be eighteen years old. His true Erelim (part angel/part human with a vampirish twist) body gives him eternal youth — and an unchanging heart — until he meets Cassie. His duty towards her means he is super-protective, while his love for her makes him want to give her the world, despite her independent streak. Alex is part of the Grayson Family — all Erelim — including Alistair, Charlotte, Amaia and Kelsey, and their trusted companion, Kaspar Weiland.

Cassie's black Labrador companion appears suddenly in Cassie's life. While an unexpected friend, they soon become inseparable, and their bond goes far beyond a normal girl and her dog relationship.

Astra has many secrets and many forms. A fallen Star she can shape-shift and has many other gifts that show up during her extremely long life.
 

Alex's sister, Amaia, is an Erelim. She appears to be around seventeen years old. Apparently open, fun-loving, and up for anything, Amaia has a well-hidden serious side, and is extremely gifted. She encourages the relationship between Alex and Cassie, and helps Cassie discover how to fit her human side into the Erelim clan. She is protective of both Cassie and Alex.

Amaia and her biological sister Kelsey have a close relationship.

Izzy is Cassie's close friend and confidante and enthusiastically helps Cassie (along with their friend Gina) to admit her feelings for Alex. The three have been close friends since the start of high school, and Izzy understands Cassie's shy nature, as she suffered bullying in her early life, too. Izzy loves fashion and accessorizing both herself and her friends' looks whenever she gets the chance.
Izzy lives at home with her mom and dad.

Gina is Cassie's other best friend. She is strong-willed and stands up for her friends when they're in trouble. She is confident, stylish, and everything Cassie wishes she could be. However, she has taken Cassie under her wing and admires Cassie's book smarts, 

Gina lives at home with her Mom, but later in the initial trilogy she moves in with her boyfriend, Tony.

Cassie's mother is a busy supermarket manager, who has a close relationship with her daughter. After losing her husband, Luke, to cancer several years earlier, she works hard to provide for Cassie and herself. She is close to both sets of Cassie's grandparents. She encourages Cassie to pursue her studies to have a more successful life than she believes she has herself. Marie is close to her sister, Bea, and brother-in-law Tom, who visit frequently.

Alex's adopted father-figure, and head of the Grayson clan, Alistair is a successful entrepreneur, and generous with his well-established wealth. While working hard, he encourages his family to follow their dreams and to enjoy their lives, while balancing the duties of their Erelim life. His sense of humour and playfulness endear him to Cassie, and Alex loves him as if he were his own father.

Alex's adopted mother-figure, Charlotte, is an independent, loving and talented woman. She works as a vet and owns several unusual species of animals. She loves to work in the kitchen and garden of their home. Alistair and the rest of the Grayson family adore her. She is kind to Cassie, offering her advice when needed. 

Jennabeth is the Head Star-Kin, and leader of the Erelim based in the USA. She appears to be in her mid-twenties, but is much older than most of the Erelim she leads. She forms a close friendship with the Graysons and is protective of Cassie. She lives in the Northern Cascade Mountains with her Star Dog, Sirius, and their guardians, Osmond, Raymond and Raylene.

Amaia's older sister, Kelsey, lives in Seattle with more of the Grayson's extended family — her adopted brothers Porter and Kal, and sisters Bianca, Jia-Li and Selina. She is important to the Graysons, but spends a lot of time working on her research, so does not appear until the third part of the trilogy. She loves her sister, Amaia, and tries to help her out whenever she can.

Alex's best friend, and an old friend of the Grayson Family, Kaspar is a jeweller who works for Alistair in one of his business ventures. He is quiet, solemn, and an excellent guardian. He assists Alex in protecting Cassie, but fears facing the Craevyrn — a group whose grasp he escaped many years earlier. 

The Craevyrn (pronounced Cray-vern) Elders, Thanatos, Sephtis, Hadria and Ubel, lead the Craevyrn — the sworn enemies of the Erelim. They are ruthless, bloodthirsty and considered evil by the Erelim. An ancient enemy, they threaten the Erelim frequently, turn unwilling victims to members of their Army, and aim to destroy those who stand in their way, in their search for power.

Cassie's childhood bully, Mallory, makes a surprising and unexpected return to Cassie's life. Hard-hearted and vicious, Mallory would do anything — including kill — to hurt Cassie. Her motives are unclear, and Cassie struggles to realize her own strength in the face of Mallory's panic-inducing presence. Mallory enjoys torturing Cassie in whatever way she can.

Tony Worthington - Gina's boyfriend. A jock with a heart of gold.

Matt - Fun-loving and warm, Matt ends up dating Izzy for a while.

Jeremy - A basketball player with an infectious smile. He loves to hang out with his mates, and develops a crush on Amaia.

Duncan & Jordanna Mountford - Cassie's close friends.

Maxilius & Elena Darkwater - Leaders of the Darkwater Erelim clan.

Maverick Darkwater - Amaia's love interest later in the trilogy.

Dr. Evelyn Darkwater - Cassie's doctor later in the trilogy.