If your character can phase matter, you need to consider a number of factors.

Does your character phase him or herself, other objects, or both?

What is the mechanism for phasing?

How much mass can your character phase?

How much surface area can your character phase?

Are there some things your character cannot phase?

How often can your character phase?

Can your character solidify either him or herself or another object inside another object? What would the effects be?

When your character phases, does he or she need to breathe?

Can your character see through the phased objects?

Can you phase some of your character and be part solid? If so, how does this work?

Example Characters: Shadowcat, Shinobi Shaw