She was a secret kunoichi (female ninja).

"Once in a lifetime" means to cherish the encounter that happens only once in one's lifetime.

He missed a once-in-a-lifetime opportunity.

He was focused on his work all day long.

No one knows what he did to spend one day.

This day was a very rare day for me.

I couldn't get in touch with her for a week.

We will meet again in one month.

Sanichi is my best friend.

There was not a speck of dirt on his shirt.