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Severus Snape is a half-blood wizard, and former Potions Master, Defense Against the Dark Arts professor, and Headmaster of Hogwarts. His portrait hangs in the Headmaster’s office along with portraits of all former Headmasters, which is heard to converse with the portrait of Albus Dumbledore. The portrait also warns James Potter not to make the same mistakes his father did and that he will be watching him throughout the school year.
Spoiler Warning: The following text provides details of the plot and/or ending of the story.

During his life, he had made numerous, small, disguised portraits of himself which he has painted in various paintings throughout the castle. Allowing for his painted self to keep watch on James Potter and his friends from different points in the castle simultaneously. He warns James that Ms. Sacarhina is involved with the Progressive Element and one of the masterminds behind the return of Merlin, using Recreant as her tool. He advises him to give the captured Merlin cloak to his father or the Headmistress. He tells the trio the Hall of Elders’ Crossing is both a place and an event—the alignment of the planets. He also warns them that the event is to occur a day earlier than Delacroix would have her students believe.