Ranmaru is a young man who was a protegee of the Oda clan. He was a loyal warrior and served as a retainer for Nobunaga during the Sengoku Jidai.
He was very young and this is why they crank his bishonen style up to eleven. He died alongside Nobunaga, so this is why he is generally his subboss on games Nobunaga is featured.
He is designed the same way as Amakusa, who is based on the historical Amakusa, a japanese born man who adopted Catholicism and became a sort of messiah in certain regions of japan at a very young age. He died during a hostile takeover of his village, as he was responsible for starting farmer revolts against the emperor for better treatment of the working class.
To those connecting the dots, Ambrosia, the demon that takes over him in the Samurai Shodown series is hinted at being the Catholic God, and Amakusa's possession and brainwashing powers are a deturpation of his missionary work, saying his faith was actually a brainwashing demon.
Go figure.