I fell in love with the MC magician, but why, I end up in the body of a secondary mob, who should die in the 2nd part of the novel?!

When Yamada Satoru read the last chapter of “I will save the world before I die”, he was very dissatisfied with the ending, at the end of which the main character died. Leaving a lot of negative comments, he fell down the stairs. When he woke up, he realized that he was not in his own body, but in the body of a secondary mob. -I will become a companion of the main character and together with him, we will go to save the world. And at the very end, no matter what, I will save him.