The climax of a work of literature is defined as being the most emotionally tense point of the plot before the falling action and the final resolution. Arguably, there are potentially many different climaxes in this novel, but to me, the climax comes when Matt finally brings his little "giftie," the "visitor" home and Kira realises that it is her father, which allows her to have the truth about the society of which she is a part confirmed. She learns the truth about Jamieson, who has been her protector, and finds more evidence to suggest that the Council of Guardians deliberately kills of people to gain control of those, like her, who have special talents that are beneficial to the Guardians themselves:
And Thomas? His parents too? And Jo's?
So that all their gifts would be captive.
This is an incredibly important point in the novel because we are left wondering what Kira will do with this knowledge and how she will respond to having confirmation concerning facts she had hitherto only suspected.