The Blue of the games is immature and competitive in the Generation I games. He strives to be better than Red and often comes off as being overconfident and cocky. He never doubts that he is superior to Red, no matter how often he gets beaten down. In the beginning Blue doesn't love or care for his Pokémon. He doesn't bother raising most of them, but instead chooses filling PC Boxes over taking care of them.
Blue's desire to be strong and defeat Red comes as a cry for attention. Blue's grandfather often neglects Blue in favor of Red. He even forgets Blue's name. Blue's hatred for Red stems from the neglect of his grandfather. He believes that by acting out and beating Red, the boy who stole his grandfather's attention, he will get the love and attention that he desires.
"My rival should be strong to keep me sharp." - Blue at the Pokémon League Championship
Three years pass in the world of Pokémon between the Generation I and Generation II games. During those years Blue matured from a bratty child to a well established adult. Blue still has the same competitive streak however his cocky and overconfident nature has become somewhat tame. Blue’s role as a gym leader doesn’t allow for him to spout insults like “Smell ya later!” Blue also seems to have lost his neediness, he no longer seeks the attentions of Red or his grandfather, but instead relies on his Pokémon to give him comfort.