Mr. and Mrs. Frank, along with their teenage daughters Anne and Margot, share a cramped attic refuge with Mr. and Mrs. Van Daan and their teenage son Peter, as well as Mr. Dussel. Other significant characters include the Dutch individuals—Elli, Miep, Mr. Kraler, and Mr. Koophuis—who risk their lives to hide the Jews and provide them with food.

In her diary, Anne portrays herself as an energetic, playful tomboy who initially feels she can do nothing right. By the end of the story, she has grown in maturity and confidence. Uprooted from her home and friends, Anne endures a nightmarish ordeal, constantly under the threat of concentration camps and death. In this tense environment, Anne is surrounded by the same adults, leading to frequent conflicts. She prefers her father's company over her mother's. "Mother doesn't understand me," she complains when her mother tries to talk to her. Jealous of the frequent comparisons to her sister Margot, Anne strives to overcome sibling rivalry. Her relationship with Mrs. Van Daan fluctuates between friendly and antagonistic. An incessant talker, Anne often clashes with Mr. Dussel, her roommate, who craves quiet. Despite the constant personality clashes, heightened by the group's claustrophobic living conditions, Anne manages to adapt to her situation.

Anne is very much aware of the outside world and listens to radio reports about the war's progress. She fears for her best friend Lies, who has been taken to a concentration camp, and for herself and her companions as the sounds of air raids and gunfire reach their shelter. To cope with her fears, Anne confides in her diary, which she names "Kitty" and treats as a personal friend.

Anne's writings reveal her strength and courage, as she retains her faith in humanity: "In spite of everything, I still believe in the goodness of man." Her optimism contrasts with Peter Van Daan's initial pessimism. Quiet and bewildered by the sudden changes in his life, Peter spends much of his time secluded in his room. Anne gradually develops a romantic interest in Peter and encourages him to reject pessimism and hope for a better future. Their dates, limited to moving from room to room, involve sharing ideas and supporting each other.

At the beginning of the book, Mrs. Van Daan appears to be an ordinary, doting mother, but as the tension mounts, she becomes panicky and neurotic. Moody and constantly complaining, she also brags about her youth, numerous boyfriends, and active social life, much to her son Peter's embarrassment. As the story progresses, she begins to nag her husband and disturb the others in hiding, arguing with Mrs. Frank over trivial matters like whose dishes to use. Mr. Van Daan, in contrast, remains reserved and tries to cover for his wife's flaws. However, when desperation leads him to steal potatoes from the others, their roles reverse, and Mrs. Van Daan tries to protect her husband.

Anne describes her family with a sympathetic tone. She portrays her mother as a quiet woman who struggles to connect with her. Mrs. Frank is perplexed by Anne's lack of natural affection and respect, qualities that Margot readily shows. Margot's kind and intelligent, yet reserved and obedient nature, stands in stark contrast to Anne's rebellious spirit. Anne's father takes on the role of leader, making decisions, enforcing rules, and providing encouragement.

Despite the bravery and selflessness of some, such as the Dutch who provide food and shelter to Jews, a recurring theme in Anne's writings is man's inhumanity to man. Anne is confined and lives in constant fear of death solely because of her religious beliefs. Tragically, she eventually...