Natsume is a extremely intelligent, to the point of being considered a genius, and despite this, ditches class frequently. Hated and feared by a lot of students at the academy because of his dangerous Alice and cold attitude. Due to the isolation he feels from most of his classmates and hatred many student feel about him he is often bullied even though he comes to ignore it, although the class he is a part of holds him in very high regard and often follows his orders.
He is thoroughly unhappy with the school and is always suspicious of the intentions of adults because he believes that they use their power over the younger students to hurt and abuse. He is involved in top-secret missions for the Academy which are shady and haven’t quite been explained in the story yet, although he often returns injured.
He is paired up as Mikan’s partner in class by the their mostly-absent class teacher, Narumi. Although at first he rebels strongly against this duty, he later accepts it and begins to enjoy Mikan’s company, along with the company of his classmates and finds himself getting friendlier as the series progresses, although he never quite loses his dangerous edge and temper.
He possesses the fire Alice. It is revealed early on that since Natsume has such a powerful Alice, it causes a lot of stress on his body to use and shortens his lifespan immensely. He had been showing the symptoms of a life shortening Alice for a year before the start of the series and had been progressing quickly due to the constant use of his abilities for missions for the Academy. Because of this, he can be seen quite a few times through the manga coughing blood or even going unconcious.
He is the only Special Star in the Elementary Division, thus entitling him to a lot of luxurious things that he never really gets to take advantage of.
The story for Natsume entering the Academy was the rumor that he was a murderer and once set fire to a village. However, it turns out that his younger sister Aoi lost control of her Alice and accidentally set the buildings ablaze, losing her Alice, memories and her sight in the process. Aoi is used as a hostage to get Natsume to cooperate when he gets testy, and even after Aoi is released, he continues to perform his missions to keep Mikan and Ruka safe.
In chapter 50, he kisses Mikan during the Christmas party, to which their relationship develops. In the most recent chapters, he searches relentlessly to find Mikan since she was taken away by the Academy for her Alice and when he finally finds her, he manages to get her to confess as a ‘reward’ for tracking her down. He does confess in return.
In the epilogue, it shows that Mikan and Natsume continue their relationship and get married.