Both Frankenstein and The Kite Runner deal with several similar themes, though separated by years of history and with very different authorship. In Frankenstein, Victor Frankenstein is appalled by the monstrosity of his creation. He is devastated by what he has done, and spends the entire novel trying to kill the monster and avenge the deaths of his loved ones.
Similarly, Amir spends much of The Kite Runner trying to escape the monstrosity he witnessed when he saw Assef beat and rape Hassan. Amir tries to make his own amends by adopting Hassan's son.
Father-son relationships are also themes in both novels. Though Victor's relationship with his father in Frankenstein is fairly typical, his relationship to his created son, the Creature, is atypical. He rejects his "son" entirely, and the Creature is left alone and parentless. The Creature becomes monstrous because he is never shown love. Victor never has biological children because the Creature murders his wife on their wedding night.
In The Kite Runner, Amir's father is ashamed of his introverted and bookish tendencies, which creates tension in their relationship, as does their move to the US. Like Victor, Amir is unable to have children with his wife, and instead adopts his brother's son in an attempt to create a family.
Because both novels deal intimately with shame, making amends, and family relationships, the theme of identity can be said to encompass all these smaller themes into a larger, unifying theme that runs across both stories.