In conclusion, with his conversation with his system. Fein's soul was a third-dimensional soul. But his vessel has the essence of 10th dimensional creatures like Niki since he was one in the past. Fein's soul was simply a third-dimensional one and has the body, which has the essence of 10th dimension.
Not all being could become 10th dimensional creature. One needs to have 10th dimensional essence to become one. So, it was almost impossible to become. After all, where you get an essence of 10th dimensional being? Unless one of them willingly gives his essence to a mortal, which was stupid.
Since Fein was a human, he has lived two lifetimes of human and experience different perspective in seeing things. Fein got the key to become an 11th dimension. No one knew that the key to ascend to 11th dimension was to experience human life and understand the essence of life. So Fein being exiled by 10th dimensional enemies was a blessing in disguise.