The one confessing their love to Subaru was Emilia's personality the whole time. perhaps the soul or personality of the witch of envy who desires love had fused with Emilia. Subaru notes that she (the black ghost hand that threatens him each time he tries to tell someone about Return by Death) seems a little off from the girl repeating "I love you" so passionately to him. It could be that Satella's jealousy towards the world and desire to destroy it has mixed with Emilia's love for Subaru, however since I'm not sure about Emilia's feelings towards Subaru at this point in arc 4 this is just a guess.