Rogue

While ability thieves appear simple and fun, I’m sure there are some guidelines that can help keep them under control.  Also, an author should be careful about making this the only defining feature of the character.  Let’s get to the hopefully helpful tips.

1. Try to put some limitations on the power.  It can be a maximum amount of powers that they have or needing time between using each one.  This way, you don’t have a character running around with god-like powers.  A hero like this would eliminate all sense of tension since it’s hard to believe they’ll fail.  A villain would seem to be an impossible person to defeat unless the powers are eliminated.
2. Clearly define how they are stealing the abilities.  If it’s by touch then that’s how it has to be for their entire existence.  They can’t suddenly switch to a new capture method without an explanation or reason.
3. Try to have a list of powers and abilities already in your head.  If you wing it, you might end up coming up with a power for every situation.  Now, you’re back to the god-like status issue.  The list doesn’t have to be revealed in the story through any method other than usage.  Simply get a sense of their repertoire instead of adding powers as you move along.
4. Remember that stealing an ability doesn’t mean immediate mastery.  The thief won’t know the exact limits, usages, and weaknesses even if they have seen the ability in action.  There can be rules they are unaware of since they didn’t get the power through training.  Since they aren’t getting the power with permission, they won’t bother asking the original owner for details either.  Trial and error might be the only way to go.
5. A character who steals abilities should be more than their ability.  Give them a personality and dreams beyond what they can do.  It can even be them hoping to live a normal life with a power that they see as cursed.  This would be Rogue from X-Men, who eventually developed romantic interests and other traits.  A power is great for a starting point, but a character needs to grow beyond it for longevity.
6. Stealing an ability or power doesn’t always include shapeshifting.  If the stolen ability has a physical trait then that makes sense.  Otherwise, you’re working with an enhanced shapeshifter who is copying instead of stealing.
7. Consider if people will like a character who steals the abilities of others.  Doesn’t usually make for a trusted ally, especially if the theft causes pain or death.  They tend to be social outcasts and are followed primarily by those who see them as fear-inducing leaders.  Characters like Rogue are fairly rare since the villain route tends to be the norm.