I tend to think that you could pull the idea of a string or a kite itself as an example of comparative language to represent the theme of sacrifice within the book. The string is a symbol introduced early on in the narrative through Amir's recollections:
I spent most of the first twelve years of my life playing with Hassan. Sometimes, my entire childhood seems like one long lazy summer day with Hassan, chasing each other between tangles of trees in my father's yard, playing hide-and-seek, cops and robbers, cowboys and Indians, insect torture – with our crowning achievement undeniably the time we plucked the stinger off a bee and tied a string around the poor thing to yank it back every time it took flight.
The string can be seen as a simile of sacrifice. This string is something that Amir is able to tie around Hassan and yank it at any given time, as Hassan would always sacrifice for Amir. For example, when Amir needs someone to run down his kites, Hassan would always do it. When Amir needed someone to serve as his audience and even someone to subjugate their own identity for him, Hassan would willingly do it. The "string" that was tied to the bee was symbolically tied around Hassan, as he would always sacrifice for Amir. In terms of a simile, Hassan would sacrifice for Amir as if a string was tied around him, something "yanked" whenever Amir would wish to do so.
I think that another type of symbol which could be expressed as either metaphor or simile would be the kite, itself. The experience of the kite was to take flight only to be cut down in competition. Kites were sacrificed in order to demonstrate personal superiority. Hassan was the kite for Amir. He was the kite for Amir. Hassan was the symbol of sacrifice in order for Amir to feel better about himself. When Amir doubted himself, Hassan always sacrificed himself, sometimes through ignorance while other times through active encouragement and loyalty, to make Amir feel better. Hassan was that kite who was cut down often in order for Amir to recognize his own greatness. Hassan actively accepted that role of the cut down kite, one who fell into an alley and hoped for someone to pick him up off the ground when Assef and the bullies confronted him. In this light, the simile of Hassan being like a cut down kite for Amir acquires a very sad and poignant significance.