Defender of the Nexus of Realities: At the beginning of the game, place a special Nexus terrain marker on the map, at least 5 squares away from any starting area. A character occupying that square can use Probability Control, even if Jennifer Kale isn't on the map. If Jennifer Kale occupies that square, she can use it in addition to her normal Probability Control.

Nexus Duo: When a friendly character named Man-Thing is adjacent to Jennifer Kale, they both modify their attack values by +1 if not already modified by this effect.

What kind of terrain is the Nexus marker?
Special terrain. It has an orange border. Special terrain has no additional properties beyond what is described in the text that mentions it (whether it is printed on a map or, in this case, the character card). So all this does is create a place where characters can use Probability Control. It is not hindering terrain, or water terrain, or any other kind of terrain beyond what is described in Defender of the Nexus of Realities.

So what happens if I place the marker on a square of hindering terrain?
Nothing. The square is still hindering terrain (or elevated terrain, or clear terrain, or whatever other kind of terrain it was before). Golden Rule #3 (the Rule of Occupancy) prevents you from placing it in blocking terrain, but otherwise it goes where you want it to go and the terrain in the square itself is unchanged.