Personality. Muichiro seems to always be forgetting things and his mind is constantly wandering. He doesn't let his emotions control him and his actions are decided based on logic. However, before his twin brother died he had a much more caring personality, showing compassion and kindness. 
Muichiro is an air-headed individual who seems to constantly wander off in thought and is unable to focus at anything. Impassive and, at times, obtuse, he tends to think only for himself and operates on logic