mccree finds out that hanzo actually loves halloween, its something that hanzo discovered after leaving the clan and being on the run, its a time of year where he can be anyone & satisfy his huge sweet tooth, so mccree devises a plan to seduce hanzo with halloween things.
the major one being his rendition of “casper the friendly ghost” by replacing the lyrics with stuff about hanzo, and he calls it “hanzo the unfriendly ghost” because he’s not gonna beat around the bush about that - hanzo isn’t mean per se, but he isn’t friendly. other lines in the song include: “the most beautiful ghost around” “likes to fright adults and children alike” “cowboys he has a heart of gold” and everyone is just like omg this is never gonna work. they especially believe this because mccree is t e r r i b l e at singing - it does not stop him.
so it works. and hanzo’s all heart eyes over this buffoon who keeps singing this stupid song, but its the sweetest and most wholesome gesture hanzo has ever received while being courted/wooed. and he insists that mccree continue to sing the song to him whenever he wants even after they get together. mccree takes time to make up or improvise other songs so hanzo can have a whole playlist.