Reen could have shapeshifted into one of her victims whose body she had eaten. Or she could have coated Eren's fingers and granted them claws so that he could dig better even without using the knives. But in the end, Reen was as immature as a year or two years old human child.
"I saw everything! His thing! It was right in my face! Are they supposed to be so big? What am I even thinking? I will kill the bastard the next chance I get!"
"You want to learn magic, right? In fact, I was there when you tested your magic talent. I happen to have a magic affinity for Dark Magic as well, and I was very powerful when I still had my physical body."