Zack and Rin are biological half siblings and have lived together since the age of four, Zack finds Rin to be extremely annoying and exhausting to deal with. Zack often acts as the older sibling, even though he is only a few minutes older than Rin. Even though he finds Rin to be annoying, Zack cares deeply for his little brother,their dynamic has become more balanced, with Zack recognizing Rin's unique talents and Rin learning to appreciate Zack's quieter,cold,psychotic and mature personality.