Katie Bell is a year ahead of Harry Potter, Ron Weasley, and Hermione Granger. She is well known for being a Chaser on the Gryffindor Quidditch team.
In her seventh year at Hogwarts School of Witchcraft and Wizardry, she had the misfortune to be put under the Imperius Curse to deliver a cursed opal necklace to Dumbledore. However, she ended up touching the necklace herself, and was cursed by the piece of jewelry, after which she was taken to St. Mungo's Hospital in London to convalesce.
Katie was an active member in Dumbledore's Army since the organization started. She fought in the Battle of Hogwarts alongside other DA members, current and former, in order to help win the war against Voldemort.
Katie was very close to Alicia Spinnet and Angelina Johnson in school, and it is assumed by the entire fan base that Katie and Angelina are best friends.
To me, Katie is a plot point, and there are plenty of ways to analyze the relationships she had with multiple different characters, such as Oliver Wood, Angelina, or even the Weasley twins, but that's another node for another day.