Kayn was never in relationship - "This being said I truly believe that Kayn didn't really have time or preoccupation for romance, but I do believe that one day he will meet a very special person who will be capable to teach him what love is."
Kayn may be a possible reference to Ling Yao, a character from the anime Fullmetal Alchemist: Brotherhood. Both characters share similarities in that a superhuman entity reside in them which vies for control of their bodies (Rhaast, a Darkin inside Kayn and Greed, a Homunculus inside Ling Yao). Moreover, Kayn shares quite a lot of similarities in character design with Ling Yao.
Kayn could also similar to Masane Amaha, a character from the anime Witchblade (An adaptation of the the comics of the same name). Both wield weapons that are sentient in nature and try to take control of their bodies (Kayn's Scythe contains Rhaast, and Masane's gauntlet contains an unnamed entity), as well as both of them possessing supernatural powers.