I've recently done an extensive comparison of different pens for writing Chinese characters (similar to Kanji). What I find the best are
1. Zebra super fine brush pen (available
here). Very easy to get an amazing range of line thickness, and feels just like a brush. Cheap! Disadvantage: not refillable, and only comes in black.
2. Richard Binder's "Condor" nib for Pelikan pen bodies. Takes some practice, but it works, and it's fun to use. Fairly expensive. A Sailor pen with a "Concord" nib would give similar results, I guess, but would be even more expensive.
3. Sailor "special script" pen (available from ebay seller greenman508). Surprisingly good for the price. Actually can make better strokes than the Condor nib in some cases.
I've tried numerous other brush pens and find them too soft, too big, and/or too hard to control.
Also, I've tried the Sailor Zoom nib, and although it's very smooth, I was unable to get it to produce enough line variation within a single stroke to achieve the appearance of brush strokes.
Finally, I've tried a flex nib, and although I imagine you could learn to use one for Asian characters, I didn't have much success.
Don