Tvrtko was not Croatian or Serbian, for Gods sake. We are talking about medieval period, then the modern ethnicities did not exist. Tvrtko was Bosnjanin of domestic Kotromanic dynasty which mixed with Nemanja and Subic family. ;)

He was genetically 1/2 Croat (because his Mother was a Croat), 1/4 Serb (because his grandmother on his father's side was a Serb), and he's 1/4 "Bošnjanin" (because he's grandfather on his father's side was a "Bošnjanin"). But most importantly, he called himself a "Bošnjanin" throughout his life, and died one.