It's a word used to describe your family member (brother)
I went yesterday with my butter to buy ice cream .
Butters
You call someone butters, if you are not attracted to them. They look like butter between your fingers feels. All gooey sticky Urrrh...
Jo Blogs to friend: "what about that girl over there?"
JO BLOGGS: "Nar, she's butters."
Mondo Owada from the game series Danganronpa
Mondo? Don’t know him, I know a butter though! (Typical fandom jokes amirite?)