I assume you already had a tablet or a big phone with stylus (hell yeah Galaxy Note!) not the useless overprice iphone. Ok now ‘à la mine!’:
SketchBook For Android:
This is a pretty good app from the famous company Autodesk, which provides a flexible tool for sketching and coloring raster graphics, especially with the stylus pen from Note. Comparing with other apps, this one is the most stable I can ever imagine, the brush stroke is very fast and the way it supports color palette is perfect.
This comes as a free app and can be upgraded to pro by a 4€ bundle (I bought it) and it’s worth upgrading, you feel like you’re working with photoshop again. (but fuck Photoshop since they Adobe have just released the for-apply-only pen and it makes me hate Adobe)
This one is different from other two above, this one helps you create vector graphic, hell yeah high quality! But it runs a little laggy and sometimes exporting to another software is a pain in the ass, anyway, for character designing, this is a pretty good application.
You have already tools for drawing raster and vector images, the final one I want to recommend is for charting: touch draw is here to do the work. This piece of shit is expensive but it can be developped in the future, the devs always say that it’s hard to develop the features in android than iOs but come on, since I also a programmer, it’s our fault if we blame the platform.
There is a few other apps, and tools that I use on Android: Ink, LayerPaint, Photoshop, PicPlay, Picsay, Photostudio…, I work also on PC with Photoshop for photo manipulation, Manga Studio for web comic, Sai and Illustrator for sketching and illustrating, and AutoCAD for deisigning but it’s another story (Yep, I list them here to showcase my ability LOL).
The most important one is still a pencil and rubber with some basic A4 paper. Let the imagination flow!