Every revenge has its consequences. 
 After Rennie's death, guilt washes over Lillia because dating Reeve secretly behind Rennie's back and their friendship didn't end well. However, Kat had a reconciliation before her death. Things went back to normal as it should, Lillia and Kat bonded but curious of Mary's unusual disappearance. They thought she left the Jar Island for good but there is something more. 
 "Revenge is sweeter than you ever were." 
 Mary is a ghost and her soul is trapped on Jar Island. She will remain unrested until Reeve got what he deserves for being such a jerk and blames him for her death and she wants him to take his own to set her soul free. Her grudges towards him is absolutely dangerous and she is capable of hurting anyone even Lillia and Kat. 
 A promising conclusion for Burn for Burn trilogy that surely leave readers a life lesson. Jenny Han and Siobhan Vivian's collaboration created such a wonderful and heart-warming contemporary and I think they should write more novels together. The story, characters and its twist are absolutely lovable. 
 Like Lillia Cho says, 
 " Life is long if you let it be." 
 because it's too precious to waste time to those who isn't worth to spend time with. In everything you do, YOU are responsible for your actions and learn how to face the consequences with confidence. There's no time machine to take you back to the past to correct things, just move on and take those mistakes constructively.