Harry Potter and the Half-Blood Prince
Affiliation: Order of the Phoenix
Tonks is an Auror who always seems to come to Harry's aid right when he needs her most. She appears distraught throughout much of Book 6. We learn that Tonks fought in some big battles last year. She even appears to feel responsible for Sirius Black's death: "Well, she was fighting Bellatrix Lestrange, wasn't she? I think she feels that if only she had finished her off, Bellatrix couldn't have killed Sirius" (5.159).
Harry notices immediately the ways in which Tonks has changed:
Now she seemed older and much more serious and purposeful. Was this all the effect of what had happened at the Ministry? He reflected uncomfortably that Hermione would have suggested he say something consoling about Sirius to her, that it hadn't been her fault at all, but he couldn't bring himself to do it. He was far from blaming her for Sirius's death; it was no more her fault than anyone else's (and much less than his), but he did not like talking about Sirius if he could avoid it. (8.31)
Harry also notices that Tonks's Patronus has changed its form. This can happen after a great "emotional upheaval," explains Lupin (16.151). We learn at the end of Book 6 that Tonks has fallen deeply in love with Lupin, thus explaining why her Patronus now resembles Lupin in the form of a werewolf.