Ice

 In book, 13 is a sad character. 13 is a killer trained since young age. 13 is not a real name, killer that trained since young age didn't given actual name but like prisoners, a number replacing naming. Even worst, 13 is actually a clone, 13 mean that he is the number 13rd clone. He is not the main character in the book, 11 is the main character. 11 is the most perfect creation of the experiment series. 11 eventually got himself a name. But 13 did not, he tried to took 11's name. And he give up. He was given up by the organisation.