Alistair 'Aly' Kershaw is a friendly, easy-going boy around Antimony's age who spends a week at Gunnerkrigg Court in the chapter A Week for Kat. The title of the chapter references the fact that Kat quickly develops a (reciprocated) crush on Aly, with whom she shares a number of interests.
From the moment Aly appears in class, it is clear there is something unusual about him: he is not required to wear a uniform, is spending only one week at the school, and does not know where he will be going at the end of the week. He also wears heavier and heavier clothing as the week passes, can jump much higher than any normal human, and yet is light enough that Annie can easily lift him off the ground.
It turns out that Aly's parents passed a test that allowed them to get new bodies, and they have transformed into birds. Aly is becoming one as well. (Kat asked him if they'd given him a choice, however his choice had been his mother shouting "Hey, Aly! Wanna be a bird?" with him calling back "'Kay" while he was playing a video game. and he is spending his last week as a human at Gunnerkrigg Court in order to be around kids his own age. As he and Kat say goodbye, Aly's transformation into a bird becomes complete, and the chapter ends with the family flying off to live in Gillitie Wood.