A Goodreads user asked:
When did Jules start to fall for Kate?
This answer contains spoilers… (view spoiler)[I believe I noticed it first in Die For Me, but it wasn't super apparent. Then, in Until I Die, there were a few moments when it became apparent that Jules had feelings for Kate. For one, he mentions how much he cares for her after the meeting with the healer. Also, with the incident in his studio, there's a moment that made me feel very uncomfortable, as I've always loved Vincent and Kate. I have not read If I should Die yet, but I would assume that his affection for Kate continues until the next novella: in which the synopsis states he might lose his heart once more for a female revenant named Ava. (hide spoiler)]