Whats wrong with Kevin?

Everyone(including myself) loves Kevin memes, but few have ever actually thought about why he's so different. 

 Ichi and Ni both have their differences. Ichi(middle head) is the "leader" of the three and more collected than Ni(right head), who is more overtly aggressive, but they both act together more and compliment each other well. On the other hand, San(otherwise known as Kevin) is shown to be more curious than his twins and even needs to be directed occasionally on what to do by Ichi. Sometimes, he even gets confused temporarily as to what his two twins are doing(see banner above)

    Did he suffer a sort of brain injury in the past? Is it just his personality? 

Tell me what you think below.

Thx Michael Dougherty!