How do I make random character appear randomly and 1 more ?

I have a couple questions and I'll try my best to explain my questions as best as I can. I have been looking around to see if someone else have same questions like mine and I may miss it by accident.

How do I make a random character/sprite appear randomly in a certain place? For example, the player decide to visit a mountain and there is a chance that the player will run into a person or mountain lion or something else. Kinda like a random encounter.

My another question: How do you lock a certain power until the condition is met and how can it be carry over to another level/layout? The best example would be Megaman classic when he defeat a certain boss and he gain a new weapon. Can it also be apply to skill tree in RPG too?

I don't start any projects yet because I have been a little busy playing around with Construct 2 and read a lot of tutorials. Thank you advance for your time and help.

1) Use a timer with a random time

2) Store it in a global variable

If your player is global you could use an instance variable on the player which you check to see if the player has a certain power, it will be carried to another level.

If the player is not global you could also use a global variable for this.

To make a random character appear you could have an instance variable/ global variable and set it to : choose("person","lion","firebreathingdragon")

and have an event if variable is "person" system create person, etcetera..