Character Attendant here.
**My vote goes to Sharpie. If you're afraid that it bleeds thru the page you can get the fine point Sharpie. It also comes in click it.
**As others have reported we strongly prefer you do not bring Green color Sharpies. Some attendants maybe more vocal about offering another color and some may prefer not to say anything afraid of a guest being irate because of it. Still our costuming department begs us not to use the green ones.
**Although most characters won't turn down a mini-sharpie they are very uncomfortable for them to use.
**Just for clarification WDW does not sell Sharpies for multiple reasons.
**The pens that they sell in the stores (the big ones) are good too. Characters like and love big pens.
**A personal plea. You can bring in anything for characters to sign. Buy a book, make a book, or sign a book we don't mind. However please don't bring those tiny notebooks. The ones that are as little as post it note pad. It is soooo uncomfortable to sign those!! Sometimes is not even enough to sign their full names!