A guide born with a physical defect—he cannot speak—but brought to the frontlines due to mission requirements. He is kind and possesses incredibly strong mental power, helping to calm the mental storms of frontline Sentinels. Despite most guides lacking combat abilities, Si Yu exceeds expectations: he can hold off a quantum cannon with one hand, take on ordinary soldiers with both, protect himself on missions, and assist in team combat by shielding Sentinels' mental barriers from enemy guide attacks. He communicates through sign language and mental clouds. He has a powerful temper when provoked, having mentally slapped Gu Nanfeng dozens of times. He is respected by all other Sentinels for his mental stability, expert technique, and gentle smile.
