One-sided love
The husband's only ability from the Death God's energy was related to the soul. He could send souls back to the death god and help Xaya with her duties as Death God's disciple. This ability became something else after he became the death god.
He could store souls and possibly bargain those souls with other gods. That was one of the concepts that Akaro and Kyoya had discussed already. They were too weak to discuss these matters with the gods, but Kyoya had developed more abilities related to the soul to not stop his development as Hybrid.
He forced all these developments, though. "It means I don't have a cure for what is going to happen to you," Kyoya smiled at his opponents, then broke his speed limits. It was the only aspect he excelled at.