[Ken isn't surprised by Nanako, exactly - he was aware of her and Chikusa approaching on some level, through his senses, especially since they weren't exactly trying to conceal themselves - but the hug from behind catches him off guard, and he cranes his head to look over his shoulder down at her in surprise.
Chikusa's question just confuses him further.] Hah?
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Chikusa's question just confuses him further.] Hah?