Modi is extremely insecure about proving himself the one to rightfully earn his father's hammer, specifically due to Thor's favouritism over Magni. As such, his rudeness and vulgarity could be seen as a result of this inferiority complex. His cocky personality was soon tinged with cowardliness, immediately fleeing for his life after the shock of his brother's death.  Both brothers also had a reputation of apparently low intelligence, as they were constantly called idiots/fools. Father is Thor.