If your character can heal manipulate probability, you need to consider a number of factors.

How does this power work?

Does the good or bad luck effect balance out?

Is this a controllable power?

How much choice of the outcome does your character have?

What are the possible effects of this power?

What is the range of effect and duration of this power?

What are the limitations of this power?

What can this power not change?

It is very important with a power like this to give a list of examples of the power functioning.

Example Characters: Longshot, Roulette