Tonks is a young adult witch
in the Harry Potter
books. She first appears in Harry Potter and the Order of the Phoenix
as a member of the Order, which is dedicated to opposing Lord Voldemort
. As such she is one of those who help protect Harry. Read on: no big spoilers follow.
She is too young to have been in the Order last time Voldemort was active, before Harry's birth, but is now a qualified Auror, one of the elite Ministry of Magic officials who hunt evil wizards. She is also a Metamorphmagus, the only known example in the books so far, which means she was born with the power to "change her appearance at will" (I'd guess to within human limits, otherwise she'd be an Animagus too).
Her full name is Nymphadora Tonks, but she calls herself just Tonks, as she dislikes her forename and thinks her mother was silly to call her that. Her father Ted Tonks is Muggle-born, but her mother Andromeda Tonks (née Black) is related to the pure-blood wizarding families such as those of Sirius Black (Andromeda's cousin) and Lucius Malfoy (married to Andromeda's sister Narcissa). On "Muggle-born": I'm not clear whether Ted is himself a Muggle, or a wizard from Muggle parentage.
Tonks is a very attractive character. Except when in disguise, she has brightly coloured hair, which she changes to suit her changing mood using her Metamorphmagus powers. She wears a T-shirt of the wizarding band Weird Sisters, and has a sunny personality. She likes to amuse Ginny and Hermione by changing her nose to shapes such as a pig snout (like Dudley's). She seems to have become firm friends with both of them, and no doubt they see her as a good role model.
When we first meet her she has "a pale heart-shaped face, dark twinkling eyes, and short spiky hair that was a violent shade of violet." She immediately changes her hair to bright pink, and at other times has it curly blonde, and long and tomato-coloured; but I assume the rest of it is her real self.
One peculiarity is that she is extremely clumsy. This is laid on rather thickly, and Rowling doesn't repeat clues for no purpose, so it must be important. Part of her training as an Auror was very easy because of the metamorphic ability, but this must have been counterbalanced by being hopeless at stealth. One must wonder whether it will have any relationship to the increasing importance of the equally clumsy Neville Longbottom in future books.
So what's her secret? Is Tonks to be a traitor perhaps? No. I have no idea what will happen in future books, of course, and Rowling is far too clever to make it guessable; but a traitor or weak link in the Order of the Phoenix is an obvious possibility. It will be a surprise, whoever it is. But in an on-line interview*, Rowling answered:
LizardLaugh: I love Tonks, she's my favorite new character. Will she play a large role in future books and/or in Harry's life?
JKR: Tonks is hanging around. I really like her, too.