Katie is the counselor of the Demeter cabin. During the War Council Meeting, it is shown that she has had a bitter rivalry with the Stoll brothers, ever since they covered the whole grass roof of the Demeter cabin with chocolate Easter bunnies. She suggests that the spy is one of Luke Castellan's siblings (thinking it is most likely the Stolls). She is similar to her mother, Demeter, when she lectures. Once, she yelled at the Stoll brothers for wanting to steal things, which is ironic since they are Hermes' sons.
Katie rode on the strawberry trucks along with the rest of the campers from Camp Half-Blood to the Empire State Building, and prepared to defend the city. During the battle she was ordered by Percy Jackson to take her cabin and defend the Brooklyn-Battery Tunnel. It was said that they "turned it into a jungle." Later on when the fighting got closer to the Empire State Building, she was heard yelling to hold their lines. After the Battle of Manhattan, Katie reported to Percy when he came down from Olympus that she saw Rachel Elizabeth Dare leaving the Empire State Building unharmed, before taking Blackjack back to Camp Half-Blood.
Katie is very kind and patient, yet passionate and willing to risk her life for the gods. Katie is said to remind Percy of her mother Demeter, by her bossiness and the way she argues. She dislikes the Stoll brothers (Connor and Travis) because they put chocolate Easter bunnies on her cabin's grass roof. As her mother is Demeter, the goddess of Agriculture, she would have liked gardening and nature.
She is shown to be stern with others, a trait she inherited from her mother.
Katie's appearance is never described in detail though she, along with the other children of Demeter, may have inherited their mother's brown eyes. She might have inherited curly brown or blonde hair and freckles. It is also highly possible she has thick black hair and brown eyes
- Chlorokinesis: As a daughter of Demeter, she has absolute control over plants. The children of Demeter helped in the Battle of Manhattan and use it to make their cabin look good.
- Culinary Arts: As a daughter of Demeter, she's a natural cook.
Katie gets along well with Percy Jackson
The daughter of Demeter gets along well with the daughter of Athena.
She has a good relationship with Grover.
Connor and Travis Stoll
She may be rather stern with everybody though, because her mother was mentioned to be rather stern. It is revealed in The Last Olympian that the Stoll brothers have somewhat of a rivalry with Katie, ever since they decorated the grass roof of her cabin with chocolate Easter bunnies as a prank. Also, she works at the camp store which unfortunately for her, is the Stolls' favorite place to steal from.
As Miranda is Katie's sister, it is assumed that they have some sort of connection.
- Miranda Gardiner is the head of Demeter's Cabin in The Lost Hero with no explanation as to what has happened to Katie. It is speculation that she is only a summer camper, because The Lost Hero happens during the winter time.
- Her last name is a clear reference to her mother, Demeter, Goddess of Agriculture.
- Katie's name was actually the name of Rick Riordan's former student named Katie Gardner.