Allan Karlsson is a fictional character from the novel "The 100-Year-Old Man Who Climbed Out the Window and Disappeared" by Jonas Jonasson. He is portrayed as a centenarian who escapes from a nursing home on his 100th birthday, embarking on an unexpected adventure that leads him through various historical events. Throughout his life, Allan has been involved in significant moments, meeting influential figures such as Harry Truman and Mahatma Gandhi. His story combines humor and absurdity, highlighting the randomness of life and the impact of one individual on world events.