Mamoru is very protective of those he cares about, risking his life to save them. He has likely suffered from loneliness and depression his entire life, and has trauma from the death of his parents resulting in memory loss (until midway through the first season). Because of this, he seemed to be very cold-natured initially and lacked many friends. He can also be blunt, which has led to him sometimes upsetting those around him due to his harsh words. However, Mamoru also has a good nature.