One strong friend relationship found in Andrew Clements's Frindle is between Nick Allen, the protagonist, and Janet Fisk.
We are introduced to Janet in the opening chapter when Nick performs a science experiment in class by chirping like a red-wing blackbird. He had learned on TV that red-wing blackbirds let out very high-pitched chirps any time they see hawks. Due to the nature of the way sound travels, the hawks are unable to tell where the chirp is coming from, so they go off hunting in the wrong direction. After noticing his fourth-grade teacher looked like a hawk, Nick decided to see what would happen if he let out a high-pitched chirp like a blackbird. Sure enough, his teacher couldn't tell who made the chirp, and she accuses Janet, who nearly cries. The narrator explains Janet is one of Nick's frequent playmates in the neighborhood, and Nick clearly admires her a lot:
She was good at baseball, and she was better at soccer than most of the kids in the whole school, boys or girls (8).
Since Janet is his good friend, Nick feels badly that she was yelled at by their teacher, so he apologizes and explains to her what he had been doing. After that, Janet joins in on the experiment, and their teacher is never able to figure out who is making the noise.
As his close friend, Janet is with Nick the day he gets the idea to call a pen a frindle. She is also the fifth kid, or "secret agent," who puts into action Nick's plan to convince others to call a pen a frindle by walking into a store and asking to purchase a frindle (34-35). As one of Nick's "secret agents," she is with him, along with other kids, when he asks them to take the oath to use the word frindle instead of pen.
Though Janet is not mentioned many times throughout the book, we can tell by Clements's characterization of her that she is a very loyal friend of Nick.