Kaitlyn

Kaitlyn is a minor character who only appears a couple of times. She is a friend of Hazel and they attended elementary and middle school together. Kaitlyn is described as having a slight English accent and acts like "an extremely sophisticated twenty-five-year-old British socialite stuck inside a sixteen-year-old-body in Indianapolis", according to Hazel. She is said to have endless social connectivity and has "the kind of packed social life that needs to be scheduled down to the minute". Kaitlyn also enjoys shopping.

Derek Wellington
Kaitlyn's current boyfriend is Derek Wellington, a minor character. While talking with Hazel, she confides she does not think the relationship will last, due to Derek's personality, and the fact he is too much of a "boy" for her tastes. Later in the book, they broke up. Kaitlyin wasn't too upset, as she told Hazel, "Oh, I got over it, darling. It took me a sleeve of Girl Scout Thin Mints and forty minutes to get over that boy."

Augustus Waters
Kaitlyn has never met Augustus, but when Hazel asks her about him, (Kaitlyn being the social butterfly) she says she would "ride that one-legged pony all the way around the corral." Kaitlyn knew Augustus used to play basketball for North Central, and she had seen him at parties. Other than that she does not talk about him or know him very much.

Hazel Grace Lancaster
Hazel Grace Lancaster and Kaitlyn are portrayed as close friends, but Hazel suggests that they're not all that close as Kaitlyn feels awkward around Hazel. The reason of this is Hazel having thyroid cancer and Kaitlyn is unsure of how to act around Hazel.

Trivia
1. Kaitlyn doesn't think that her relationship with Derek will last because he is such a 'boy', this implies that she prefers 'men' (i.e. people more mature than her).

2. Kaitlyn was cast in the movie, but her part was edited out of the final.

3. Since the story is narrated from Hazel's point of view it was difficult for John to show the readers how beautiful Hazel was and how popular she was during her stint at school so he wrote about Kaitlyn who is obviously extremely popular and beautiful. John Green wrote on his website that he really liked Kaitlyn's character, and that he intentionally made Kaitlyn somewhat shallow and not deep enough because he wanted to contrast between her and Hazel's personalities and subtly show that beautiful and popular people like Hazel can be intellectually engaged too.

External sources
Mini TFiOS Wikipedia biography www.johngreenbooks.com