User blog comment:Crimson and Darkness/Volume 25 discussion blog/@comment-31764731-20180326095200/@comment-31984046-20180327145602

In vol 12 when Ajuka discoverd that 4 of Issei's evil pieces mutated he said this:

“Out of the eight, four of the evil pieces have turned into [mutation pieces]. Though there are different values on each of them." So we know that a mutation piece from person A and B have different values and one can hold(give?) more power than the other. So if DxD G continue to raise Issei evil pieces value(The power of infinity is slowly changing his body and soul, in vol 24 in his fight with Thanatos when he was cut by his scythe he didn't feel a thing even thought Thanatos is God-class. Even in CxC Thanatos couldn't cut his soul.) then there'll be no need to worry.