The relationship between the hybrid doppelgänger, Katherine Pierce and tribrid doppelgänger, Alexandria Gilbert.
When Katherine first came back to Mystic Falls in 2010, she was jealous of Alex for earning the affections of both Stefan and Damon, as she had also had relationships with both brothers in the past and, while she never truly loved Damon, she was still in love with Stefan. Katherine also planned to deliver Alex and her twin to Klaus for the sacrifice in exchange for his forgiveness for going on the run for him for 500 years after ruining his first attempt at breaking his Hybrid Curse.
They are referred to as "Kathlex" by fans.