I believe all characters sign autographs, BUT at WDW some of the characters sign during certain times of the day. For instance, we wanted to get Snow White's sig and missed her by minutes because she signed between 9:30-10:00. She would again sign at a later time in the day--unfortunately, not a good time for us. We should've gotten there sooner. Lines for character signatures can be long. Make sure you allow for that, too.
Guest relations could probably give you the character signature times and/or appearances.
Hope this helps!!