Death Note: Amane Misa's Lifespan (Spoilers)

Unlike Light, Misa as the second Kira has the Shinigami eyes, allowing her to see the name and remaining lifespan of a person just by looking at him or her. She obtained the eyes by making the a deal with Shinigami, trading half of her remaining life span in order to obtain the Shinigami eyes. She did this twice, once with Rem, and once with Ryuk.

However, this does not mean that Misa will necessarily live a short life. At Misa’s introduction, Rem states that a Shinigami named Gelus saved Misa’s life by killing a potential murderer. The Death Note rules state that Gelus must die for extending the life of a human, and that whatever Gelus’s remaining life would have been gets added to Misa’s lifespan. It is after this when Misa made the trade for Shinigami eyes. So Misa’s remaining life span becomes: ((Misa’s original remaining lifespan)+(Gelus’s remaining lifespan))/2.

When Misa temporarily loses her memories according to Light’s plan to prove both of their innocence, Misa loses her Shinigami eyes. She realizes that she does not remember L’s real name, meaning she is unable to kill him for Light. Thus, Misa does the eye trade a second time, hoping to meet L again and find out his true name with the intention of finally killing him. Thus her life span becomes: ((Misa’s original remaining lifespan)+(Gelus’s remaining lifespan))/4.

When it is apparent that L is on track to finding out Misa is the second Kira, Rem realizes that L must die in order for Misa to be protected. Thus, Rem sacrificed her life to save Misa from L by killing him with her Death Note, which is exactly what Light wanted to happen. Thus, Misa gained Rem’s remaining lifespan as well.

Thus, Misa’s final remaining life span becomes: ((Misa’s original remaining lifespan)+(Gelus’s remaining lifespan))/4 + (Rem’s remaining lifespan).

It is impossible to know for sure whether Misa’s lifespan increased or decreased as a result of interactions with Shinigami. However, it is likely that her lifespan actually increased, and by a lot. A Shinigami’s remaining lifespan at any given point can range from a single day to hundreds of years, more likely the latter.

At the end of Death Note, we see Misa wearing black (a sign of mourning of Light’s death) while riding an empty train. Later, we see her staring off into the distance while standing on an empty roof of a building. These scenes symbolize that she will live an extended life – that she will remain alive in the world when all of her friends have passed away. This implies that Misa will indeed live for a very long time.