Invisible
Very simple. In the Event Commands dialog, tab #2, right side, there's a button named "Hero Sprite Transparency". Click it and set the desired state. Remember to set it back when you're done with it! For semi-transperancy, see below.

Monster
Make sure you have the charset you want, first. If you don't, look for one online. Once that's settled, go to the Event Commands, Tab #1, right side, and hit "Change Sprite Association" and pick the character you want to change, and the "Change Sprite" button. This is also where you can set semi-transparency (a ghost effect).