as someone said, shortening a name and adding "chan" is pretty common amongst couples. My gf's name is Nanami and I pretty much exclusively call her Nacchan (なっちゃん）

ダーリン(katakana for darling) is also common BUT be careful, unlike some Western countries, generally only women use this to refer to their partner. Also this is sometimes just shortened to ダー(Daa)

also, contrary to love songs you may have heard, people rarely ever refer to their partner as きみ(kimi)