Yasunari Adachi was Masako's other son. He was hanged alongside his brother Shigesato after Komoda Beach, and Jin helps Masako find and bury them both in her tales.
You find his body with his brother's on a tree at Komoda Beach in A Mother's Peace. His death, and the hanging that denied him an honourable end, feeds Masako's rage through her tales.
Masako Adachi
Lady Masako Adachi is the only survivor of Clan Adachi and one of Jin's allies. Her husband and sons die in the Mongol landing and the rest of her family is murdered the same night. Her side storyline, Masako's Tales, has you hunting the people behind that massacre.
Shigesato Adachi
Shigesato Adachi was Masako's son. The Mongols hanged him and his brother Yasunari after Komoda Beach. Masako has his armour repaired and gives it to Jin as the Samurai Clan Armor.