João Manuel
Prince
Prince of Portugal and the the eighth son of King John III of Portugal who would have inherited the throne if not for his death from illness.
He survived his four younger siblings, who died in childhood, but was constantly sick growing up.
He married his first cousin, Joanna of Spain.
He died before the birth of his son, who would become King Sebastian I.
He was heir to his country's throne, like Prince George Alexander Louis of England.