She eventually caused an uproar amongst the students, and when it came time to fight back, Dean was one of the first to arrive for the inaugural meeting of the group that became Dumbledore's Army in 1995, and his support for his friends does not appear to have faltered, even though Seamus, his best friend, initially believed the smear campaign the Daily Prophet launched against Harry and Albus Dumbledore over their claims that Lord Voldemort had returned (at least until an interview by Harry for the Quibbler brought Seamus back to his senses).
Also that year, Dean accidentally insulted his new Divination teacher, Firenze, after Umbridge sacked Sybill Trelawney. exams, and after he finished, he was pleased to hear that the results would not arrive until sometime in July, meaning he did not have to worry about them until the middle of the school holidays. Some time around the end of his fifth year, Dean became romantically involved with Ginny Weasley In his sixth year Dean replaced Katie Bell as the Chaser for the Gryffindor Quidditch team.
Also in this year Dean would take on the subject Divination and was, along with Lavender Brown, one of the few students in their class unfamiliar with the Grim.
However, he hung a West Ham football team poster above his bed, much to the bemusement of Ron, who could not believe Dean would find a game with only one ball, two goals, and where no one was allowed to fly, interesting, prodding the poster in an attempt to get the football players to move.Dean, being artistic, drew a banner entitled "Potter for President" for his dorm-mate Harry Potter during the latter's first Quidditch match.In his second year, Dean Thomas joined the Duelling Club, a precautional club taught by Gilderoy Lockhart in an attempt to train students in battle against any possible danger that may be lurking around Hogwarts.He, among other students, shared their speculation about the Chamber of Secrets and what might be inside and how to get in.After the third and final task, Harry returned to Hogwarts with Cedric Diggory's dead body, and he said that Lord Voldemort was at large again.
Albus Dumbledore took this information and at the end of the year he gathered students for a Memorial feast to Cedric Diggory in which he told them about the events that took place during the third task.Dean survived the Second Wizarding War and was seen after the final battle sitting next to Seamus Finnigan and Aberforth Dumbledore in the Great Hall. His father left when Dean was very young, never telling his wife that he was a wizard, out of the belief that not knowing the truth would protect her and their son from the potential dangers of the First Wizarding War.The arrival of an acceptance letter for Dean from Hogwarts caused Mrs Thomas to suspect that Dean's biological father may have been a wizard.Dean was forced to go on the run to avoid arrest and imprisonment by the Muggle-Born Registration Commission, missing his seventh year of schooling.He was eventually captured by Snatchers, but escaped Malfoy Manor along with several other prisoners and soon returned to Hogwarts School of Witchcraft and Wizardry to participate in the Battle of Hogwarts.Dean was about to make a correct speculation, but Professor Binns cut him short before he could finish.