Tonks is one of the few twenty-something witches we encounter in the Potterverse.
Despite a reputation for "improper behaviour" that kept her from becoming a Hufflepuff prefect at Hogwarts, she aced enough N.E.W.T.s to qualify for Auror training with the Ministry of Magic. She was born with a rather uncommon magical trait - as a metamorphmagus she could vary her appearance at will. Going about with purple hair on a daily basis? Totally doable. Entertaining teenagers at the dinner table with a porcine snout? No problem! Disguising herself to go undercover for Auror duties? Her shape-shifting abilities more than made up for the innate clumsiness that almost led her to fail her Auror exams.
Named Nymphadora at birth, she quickly decided she was only going to answer to her last name. If she were an FBI agent working on The X-Files, she'd say, "I even make my parents call my Tonks," but her father Ted certainly got away with calling her Dora. Her mother Andromeda had been disowned from her family, the Blacks, for deigning to marry a muggle-born but Tonks didn't seem to mind that at all.
Tonks had considerable coolness points in the eyes of Harry and his friends. She had PURPLE HAIR, dude! And she turned it bubble-gum pink when she felt like it. And she goofed around and acted silly, but was ace as an Auror. She was one of a handful of wizards who accepted that Voldemort had returned, and defied the Ministry to provide information to the Order of the Phoenix while Voldemort and his Death Eaters entrenched themselves.
I'll bet Harry wanted to be Nymphadora Tonks. He sure didn't want to be Harry Potter that year.
Tonks also fought in several battles throughout the Second Wizarding War, including those of the Department of Mysteries, the Astronomy Tower, and the Seven Potters.
After her spectacular debut in the 5th Harry Potter book, Tonks was sadly marginalized in the series. She had a mostly-behind-the-scenes romantic relationship and eventual marriage with Remus Lupin. Lupin's lycanthropy was a major point of contention between them. Lupin considered himself dangerous and unworthy of Tonks, while she persisted in convincing him of her loyalty.
In the final book, Tonks handled her pregnancy alone while Lupin fled and joined the war effort against Voldemort. They reunited shortly before the birth of their son Teddy Lupin.
A month later the Battle of Hogwarts took place. Initially in hiding with her mother and newborn son, she joined the battle and fought back to back with Lupin until she was slain by her aunt Bellatrix Lestrange.
Nymphadora Lupin (née Tonks)