Not sure how to translate this in a way that fits the speech bubbles, but An goes 'Ahhhh', then Kohane responds 'I', which is the second letter of the Japanese 'alphabet' after A. But instead of continuing by saying 'U' which is the next letter, An says 'shi', so it's starting to form the word 'aishiteru' (I love you). Then Kohane responds 'te' and An finishes it with 'ru'.