They're also in same universe as passion. A side couple, Im not sure if they've made an appearance yet though. They're the 2nd couple in diaphonic Symphonia (which is like the next novel after Passion with Ilay x Taeui)
Here's the novel description:
Jeong Taeui, who suddenly became a fugitive because of Ilay, unexpectedly finds his life as a fugitive at the RieGrow household quite peaceful. However, this peaceful life doesn’t last long. Christoph, Ilay’s childhood friend and an absolute beauty, visits the RieGrow household and takes a book cherished by Kyle. At Kyle’s request, Taeui heads to the Tarten in Dresden to retrieve the book.
In the midst of a fierce succession battle to determine the heir of Tarten, Taeui meets Christoph, who seems to have strained relations with everyone in his family and is in a deadly rivalry with Richard, the most likely candidate for heir. To make matters worse, the dazzlingly handsome Christoph openly shows his affection for Taeui in an awkward manner, and as Richard watches this, he begins to experience feelings he himself does not understand…

The art is leveling uppp before my eyesss its a similar improvement to passion, their face cards are so much more dimensional. Also i find it funny how this author has the best couple, the worst couple, and the most polarising couple in kbl in a single universe.