Someone you have to watch out for. Unpredictable. People named Ronnie tend to be prone to random outbursts, obnoxious singing (with the wrong lyrics), stealing people's shampoo, popping wheelies in a kayak, and in general making no sense. But he's funny as mess.
Dude: "Yeah, I'd love to hang out but my cousin Ronnie's coming to town, I'm gon' hafta hide my shampoo and everything"