Word for someone or something deeper than most would assume

I was looking for a word to describe someone and I'm certain it exists... to describe a person is usually assumed to be one-dimensional but is in fact not, or even something more literal like a deceptively deep body of water. Does anyone have any suggestions?

You can consider:

there's more to someone/something than meets the eye

A person or situation is more complex or interesting than they appear. (Oxford)

It's an idiom, not a single word. But it applies to both someone and something, as required.

Maybe 'still waters run deep' or 'there's more to a book than its cover'.

[+amgD]

"Sleeper."

I didn't know of this term until 1996, when I was working towards my CSPA A-CoP (most basic level of skydiving "licence" in Canada, issued by the Canadian Sport Parachuting Association), Buck Whalley--operator of "Mile High Parachuting" used this remark referring to my "until-then-beknownst-even-to-myself" skydiving-related aptitudes.

When I asked what the term meant, he said it to refer to a kind of car, such as a "muscle car that doesn't look/appear at first glance to be one of any remarkable power" (my own paraphrasing).

I don't know of how common or uncommon it is as a colloquialism in English, but perhaps that makes "Sleeper" itself a "sleeper" when it comes to terms q;-)

(As an epilogue, I've since had two cut-away malfunctions that could have been vastly mitigated with more care on my part and haven't skydive'n since 2005... q:-D)