Emma symbolizes Grey's emotions. Whether everything is going great for Grey, or he's in a gunfight with Am-west, he is always thinking about Emma. If she is not with him he worries about her and that effects his mood and how he feels. When Grey and Emma scale the wall, Grey realizes how much he truly loves Emma. When Emma gets put in prison, everything Grey does is to try and get her out. His whole plan was to defeat Frank and run away with Emma and start a new life with her.
Frank symbolizes the evil/bad in the book. Sometimes people aren't what they seem. When Grey was rescued by Marco, he was taken directly to Frank. Grey trusted Frank because he told him what he thought was the truth. It turned out that Frank was manipulating Grey. Grey later found out that Frank was lying and he ran off to find Harvey so he could fight against Frank and not let him take over the world. Grey was very disappointed when franedturned be a bad guy. Grey was ashamed that he couldn't tell that Frank was lying, and that he almost fell into his trap.