Being a vampire, he was a fast runner which Kathryn must have totally forgotten about or did she? because the moment she reached the beach to stop this coughing and difficulty in breathing he was right behind her.
But Lucas didn't approach her, he watched her take her clothes off one by one as she coughed, he knew it wasn't right to watch her undress and cough and invade her privacy but he was selfish at the moment, he wanted to behold her nakedness again as her ass looked so soft and beautiful.
So, the vampire stood behind still watching her take her clothes off, he soon realize that she was never wearing a bra and on this cold night. Why was she undressing? Was she planning to masturbate in the open? No, she perhaps might want to swim but who swims with no...she probably has her own taste and she was thinking no one was watching her.
