When the Titans almost broke through Wall Maria, he and the rest of the scouts went into battle. You're a titan shifter, not intending to harm people, so you just found yourself under a hot hand of Erwin. He saw you and rushed into battle, in turn you began to run away, not knowing what to do

But having opened nape of your neck with sword, he saw how you were enveloped in thick steam, and in the place where the titan been, you're lying unconscious. He is shocked by what he saw