Soda, the middle of the Curtis brothers, generally has a positive effect on other characters. He is laid-back, handsome, friendly and cheerful, and therefore gets on well with people. In a word, he is charismatic.
Soda's most important relationships, though, are with his two brothers. He is more open and easygoing than either of them - Darry, the eldest is serious, always conscious of his responsibilities, while Ponyboy, the youngest, is rather nervous and unsure. The two of them continually spar with one another, with Darry trying to instill a sense of maturity into Ponyboy and Ponyboy resenting what he feels are Darry's heavy-handed ways.
Soda is caught between his two brothers and has to play the role of mediator; therefore he is the middle brother in more than one sense. It is not until near the end of the book that the other two realize what a strain this puts on Soda. Because he always seems cheerful, other people generally assume that he hasn't got any problems, but he finally breaks down and runs away from his brothers when they're having yet another quarrel. He confesses that how badly it affects him, being caught in the middle all the time. The revelation shocks and even 'shames' Ponyboy:
Neither of us had realized what it was doing to Soda to hear us fight. I was sick and cold with shame. What he said was the truth. Darry and I did play tug of war with him, with never a thought to how much it was hurting him.
Soda, then, acts as peacemaker and mediator. He is part of the Greasers' gang like his brothers but generally he is not into any kind of conflict. He is very generous in spirit towards other people, but that doesn't mean he hasn't problems of his own. We only really see his emotional vulnerability at the end of the book (when he also loses his girlfriend Sandy). Ponyboy simply idolizes him but also has to come to a more mature understanding of him - as, indeed, does Darry. When they both realize how much they've been hurting Soda, it has the effect of bringing the three brothers closer than they've ever been.
As the middle brother, Sodapop Curtis lands the unfortunate job of acting as the buffer between his serious older brother Darry and his younger, spirited brother Ponyboy. With his easy-going spirit, Sodapop often knows just how to handle uncomfortable emotional scenes between Darry and Ponyboy; for example, Sodapop soothes Ponyboy after the frightening ordeal of being jumped by Socs, by reminding him "Easy, Ponyboy. They ain't gonna hurt you no more" while Darry, made uneasy by his little brother's crying, stood by awkwardly "with his fists jammed in his pockets" (Hinton 8, 9). Sodapop affects his brothers by being nurturing enough to understand exactly what each of them needs--Ponyboy needs someone to listen to him, while Darry benefits most from having someone remind him not to be too serious all the time. Ponyboy observes that his brother Sodapop "isn't afraid of [Darry] like everyone else and enjoys teasing him" and "for some reason, Darry seems to like being teased by Soda" (Hinton 9). Sodapop's intuition and loving spirit helps hold his family together; he actively works on nurturing and encouraging each of his brothers in the was that best suits them.
Sodapop is the middle brother in the book, which always tends to be an important role in books and real life. The middle child tends to go somewhat unnoticed, and in his case, he makes up for that with his charismatic personality. He is the type of character that just genuinely gets along with everyone and can make any situation better just by being there. He is a positive role model for Pony and sometimes acts as the peacemaker between his younger and older brother.
Soda or Sodapop Curtis affect other character like Ponyboy, Steve, and Darry. Sodapop sticks up to Ponyboy whenever someone is causing problem, same for Darry. He is also the one that brings Ponyboy wherever he goes if he is willing to. He is also Steve's best friend which might affect him.