Kanon is Leon's cousin and the closest person to him, resulting in her being kidnapped as his Captive for the first motive of the Killing School Life.
The two had a sibling-like bond since childhood, though Kanon didn't see it this way and is in fact deeply in love with Leon. She would do anything to get Leon to notice her, having stalked him and constantly pushing her feelings of love onto him. She constantly confessed her feelings to him, but he always responded he just didn't like her that way and comforted her about being rejected. He also had a lot of belief in her, believing that she could throw a 160 meters per hour ball, despite the fact that the Japanese record for women was 140.
When Kanon finds out that Leon had died due to the members of the Future Foundation, she swears revenge. When she discovered he had died, she first tried to kill herself, and wondered why she wasn't dead already, since Leon was her world and life.
In Chapter 4 of Ultra Despair Girls, Komaru Naegi can find Kanon's notebook filled with "Big brother Leon" written in it on several pages.
In one event of the bonus mode in New Danganronpa V3, Leon mentions that he has sometimes thought about bringing his cousin a souvenir for the family's New Year's meeting. He asks Kirumi Tojo's help to plan the gift, though he says she would be pleased with any gift. He also mentions being a good "big brother".
The ship isn't that much popular due to its originating from a light novel Ultra Despair Hagakure rather than one of the main installements of the series, as well as due to barely being referenced in the main installements. The ship is preferred as a family ship because of Leon and Kanon being cousins.