I think that you are sexy.
= 넌 섹시한 것 같애.

More and more words in English is used as its original forms. So is the sexy. Generally, Koreans do spell "섹시" and sexy in daily life.

But, sexy is not a noun, so we add '한' to make 'sexy' to "섹시한."

Regards,

I fully agree with the answer. But I'd like to emphasize on the issue of preserving original Korean words, avoiding borrowed words as possible as we can do. So much more Korean-like expression will be the former ones.