2D Man would look like a sheet of paper (let's say more or less human-shaped) with various internal organs printed on it, when viewed from the side. When viewed from the front he would be invisibly thin. Let's postulate that because of his other-dimensional origin he can't be torn or injured by physical means (otherwise the first fly that ran into him would poke a nice hole...) Also he could instantly slice through any 3-D object.

Let's see, he has to have some character-building weaknesses... He's homesick for the stability and normalcy of his old 2D universe, he can only see in a line that falls in the plane of his thin direction (his eyes would be built that way), and he has a debilitating fear of Tuvan throat-singing...