Daniel explains that Charlie stopped liking him when Daniel was six and Charlie was eight. Daniel hadn't yet figured out that he was just an annoying younger brother, so when Charlie rode off on his new bike with his older white friends, Daniel chased them. Finally, Charlie hopped off his bike and stood waiting for Daniel.
This vignette shows that at one time, Daniel idolized his older brother and desperately wanted a relationship with him. In the present, this rejection indicates that Daniel still likely holds this lack of affection against Charlie.
Daniel had tried to call Charlie "hyung," a Korean title for younger brothers to use to address their older brothers. When the word slipped out, Charlie turned red with anger. His friends teased him about a "secret Korean code," and Daniel watched Charlie clench his fists. When Charlie told him to get his own friends and stop following him around like a baby, Daniel wished that Charlie had just hit him instead. Charlie grabbed his bike, informed his friends that his name was Charles, and pedaled away. Daniel never used the word "hyung" again.
Notice that Charlie isn't just rejecting his brother; he's also rejecting his Korean heritage by rejecting "hyung" and stressing his American first name to his white friends. This suggests that Charlie is ashamed of his heritage and thinks that he needs to shed it in order to fit in and thrive in America. His friends’ taunting suggests that he might not be wrong about feeling so much pressure to assimilate.