The Patronus defending against the AK makes sense in terms of what we know about the two spells, as well as the evidence from Azkaban, as others have noticed. Also, the Patronus responded to Harry’s desire to protect the Auror—the response of the Patronus seems more likely if it was a property of the spell rather than of the magic of the person casting the AK (weak evidence, sure).
Slitherin --> Slytherin
The Patronus defending against the AK makes sense in terms of what we know about the two spells, as well as the evidence from Azkaban, as others have noticed. Also, the Patronus responded to Harry’s desire to protect the Auror—the response of the Patronus seems more likely if it was a property of the spell rather than of the magic of the person casting the AK (weak evidence, sure).