Leave it to the Japanese to create a brush pen. This pocketable pen has a super fine brush tip of actual bristles, perfect for tiny Kanji characters, or of course, doodling in your journal, or sketching in your Moleskine. While it's hugely popular with comic book folks and cartoonists, artists of all stripes have picked one up for their paper work. The feel is incredibly tactile and lovely. It works like a fountain pen, with replaceable rich ink cartridges. Once capped it doesn't leak as far as I can tell. (There's a moment of panic when you first assemble it since the instructions are 100% in Japanese, but just insert the ball-bearing end of the ink capsule into the tip.) You can purchase other color inks as well.
Pentel Pocket Brush Pen
Available from Wet Paint Artists' Materials