Harry and Tonks immediately took a liking to each other, and Harry was particularly impressed by Tonks's position as an Auror and extraordinary Metamorphmagus powers. Tonks was impressed with Harry's Firebolt and tried to comfort him during his trying times with the Ministry of Magic. Adopting a big sister role for him, Tonks was greeting him by saying "Wotcher, Harry".
Later, during the Battle of the Department of Mysteries, Tonks was among the Order members that came to the rescue of Harry and his friends. Harry was later relieved to hear that she would recover from the injuries she suffered at the hands of Bellatrix Lestrange.
Tonks' friendship with Harry grew for the next two and a half years. Harry hoped Tonks would be among the Aurors set to escort him to the Hogwarts Express, though she sadly was not. Shortly after, Tonks rescued Harry on the Hogwarts Express; she performed the counter-curse to remove the Full Body-Bind Curse placed on him by Draco, healed his broken nose, and ensured he reached safety within Hogwarts. She could do this because she had been watching the students exit the train and immediately went to investigate when Harry had not gotten off too, indicating her protectiveness towards him despite her heartache. Harry was disappointed at how gloomy Tonks had become, missing his friend's bubbly nature.
When she married Remus Lupin, a happier Tonks was sorry that Harry couldn't attend the wedding due to him being required to remain with his Muggle relatives to ensure the protective charm on him held. Later, when Lupin briefly left Tonks, due to fears that the child she carried would be infected with lycanthropy like himself, Harry was furious with him for leaving his family and wanting to leave his friend to raise a baby without its dad. It was largely because of Harry's words that Lupin returned to Tonks. When she gave birth to a son named Teddy Lupin, both Tonks and Lupin agreed that Harry would be the godfather. Harry was devastated when he learned that Tonks was murdered by her aunt, Bellatrix Lestrange. He noted that the grief he felt over the death of Tonks was akin to a physical wound.