Secret, A Secret
noun, な adjective, の adjective
Secret and secrecy. Combine the two and you have quite the secret.
ひみつ
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
秘密は誰にも言わないよ。ちゃんと、口にチャックしておくよ。
Your secret is safe with me. My lips are sealed.