Mantis

A giant praying mantis. What else is there to say?

A very, large, as in 6 feet tall preying mantis. Red eyes, green chitin, you know.

Mantis is very smart, and likes to study human interactions.

Being a praying mantis, he can actually fly. (Yes, they can fly) Not very far, but he can do it. He also of course has the claws that his species is so well known for.
He moves insanely fast. I mean, blink an eye and he's waaaay off in the distance. He reacts nearly as fast, as well.

Mantis isn't actually sure what made him so large, fast, and smart, but he's willing to go with it. Humans are very, very interesting to him, much more interesting than returning to a smaller size and being eaten by some random mate. So he explores, investigating human and nonhuman alike. He doesn't really have much else to his past, although he is (wrongfully) suspected of random murders sometimes. He's actually very peaceful.

Mantis is actually a bit afraid of females. He has good reason to be!