- "No! I don't care! I am a part of Dumbledore's Army too! If you're going, I'm going; there is nothing you can do to stop me!"
- —Dennis' determination to partake in the Battle of Hogwarts
During school, Dennis looked up to his older brother, emulating him in many ways; garnering a love for photography, an almost permanently cheery attitude and showed incredible enthusiasm towards Harry Potter and Dumbledore's Army. After the events that transpired at the end of Dennis' third year; the death of Dumbledore and the takeover of Hogwarts by Death Eaters, Dennis and his brother were pulled out of school and went into hiding. While in hiding, Dennis and his brother tuned in to Potterwatch and listened for updates on the war effort. When Dumbledore's Army was recalled, Dennis returned to Hogwarts along with Colin, despite his family's wish to stay behind. Although initially sent away by Professor McGonagall, Dennis followed Colin when he snuck into the Room of Requirement and fought in the final battle. At the Battle of Hogwarts, Colin was murdered by Death Eaters, which made Dennis scream in agony upon witnessing Oliver Wood carry his brother's body into the Great Hall.
The sights and experiences at the Battle of Hogwarts would haunt Dennis for years to come as after the war, he developed a case of Post Traumatic Stress Disorder. Although he returned to Hogwarts in 1998, his many nightmares and flashbacks to the horrible battle due to his PTSD made him withdraw from the school and seek medical help. He would do years of therapy with Wizarding Psychologist Alyssa Halkirk, whom he would later marry and have two children with; Colin Angus Creevey, in memory of his deceased brother, and Brendon Ethan Creevey, in memory of Alyssa's deceased brother. Although Dennis never returned to Hogwarts, he became a well-known photographer with his own business and was hired to take photos for many famous magazines.