Sissel (also known as Yomiel's Fiancée) is a minor and unseen character who is mentioned in Ghost Trick: Phantom Detective.
WARNING: Spoilers may follow!
Yomiel's Fiancée is one of the individuals who was deceased during the course of the game. When Yomiel regained his body and found her, she was already dead, leaving a note saying, "I'm coming to you, Yomiel." This clearly implies that she killed herself to join her fiancé in the afterlife after figuring out that Yomiel passed away. However, she wasn't aware that Yomiel was still "living", due to the radiation of the Temsik fragment.
It's later revealed that Yomiel named the protagonist after her.
Her death is negated because of the timeline reset and she came to see him after his prison sentence was over.