This review is by Kalina Wilson, who can also be found at geminica.com.
I'm a fan of Kuretake's gray Fudegokochi pen as a tool for adding a water-soluble middle value to sketches. Here's another option for adding middle values: the Kuretake Brush Writer in light gray ($7.50 at JetPens). I had been considering these pens for awhile, but finally pulled the trigger after reading Stephanie Law's review.
The Brush Writer line offers a real brush tip set on a squeezable plastic body filled with water-based dye ink.
I tested this pen in violet as well as light gray in order to explore the "blendable" functionality, though if I buy any more of these it'll be the gray and blue-gray since violet and the other colors seem too highly saturated for regular sketching. When will I ever want to draw a landscape or portrait in bright, straight-ahead purple? I'd go with Copic's wine, but... not this purple.
Here's the really great thing about the Brush Writer: the tip is an actual brush, not molded felt, and in fact it looks identical to my excellent Kuretake No. 13 Hair Brush Pen.
Comparing tips, top to bottom: Kuretake Fudegokochi gray pen which is truly a pen rather than a brush, Kuretake Brush Writer in light gray, Kuretake No. 13 hair brush pen loaded with a dark gray ink, and Pentel Color Brush in black.
In the sample above, it looks like the light gray ink in the Brush Writer is significantly lighter than the Fudegokochi. Below it seems just slightly lighter. This is all due to the Fudegokochi's variability- it is lighter on sized watercolor paper than on, say, plate bristol. Generally, though, the Fudegokochi is a little darker and warmer than the Brush Writer in light gray. Both pens wash out with water quite easily, though washing over the Fudegokochi will often leave a trace of your original line.
The body of the Brush Writer looks a lot like the Pentel Color Brushes which are widely available in the US. My partner reported that he doesn't use his Pentel Color Brush because he was always hitting feast or famine with it - it would run dry, he'd squeeze the body for more ink, the tip would flood with ink to the point of being unusable, and then as soon as that was cleaned up it would be dry again. When I tested his Pentel brush, that's exactly what it did for me as well. I had no such problem with the Kuretake Color Brush. However, when I did experiment with squeezing the body for increased ink flow...
...the results were very different in the two Brush Writers. The light gray brush did exactly what I wanted - it just got a little wetter, enough to change the line from brushy to solid. The purple brush flooded so I got a blurb of ink on the brush tip and it made little lakes on the surface of my paper. When I squeezed it again, it seemed like ink and air bubbles were practically dripping out of the base around the brush tip. The only Pentel Color Brush I have ever used behaved just like this purple Brush Writer when squeezed and it required constant squeezing to achieve ink flow, so even with this inconsistent behavior the Brush Writer is ahead... I just wish I could tell you they would all act like the light gray brush, because that one is spot on. While I am going to recommend this pen, it would be a good idea to test yours first on scrap paper to see if your ink release mechanism is leaky.
The light gray pen had no problems. The tip offers good control but the loose feel of a brush, and ink flow is good and controllable. Use it over waterproof ink as a convenient form of gray watercolor. Let it dry, and you gain the ability to layer for a darker gray (though it never becomes waterproof). I sometimes keep Noodler's Lexington Gray in my brush pen for this kind of work, but for sketching it's great to have a water soluble gray as well and this is a good one. Sometimes I wished it was a bit darker, but this is "light gray" so perhaps I'll give the regular gray pen a try.
If you use the Brush Writer over other water-soluble inks or watercolor, it melds with them beautifully. It was a lot of fun to use the pen this way.
As mentioned above, I got the Brush Writer in purple as well in order to test blending. It worked fine - the drawing to the right was made with the purple pen, then the light gray was used to blur the purple into shadows - but this techniques works fine with other water-based inks too so I'd just as soon abandon the purple in favor of a fountain pen loaded with an ink of my choice.
The Fudegokochi is great as a pen that works well for line drawings in gray that can be wetted down to simulate watercolors. The Brush Writer has a different role - it won't write like a pen as well as the Fudegokochi, but if you want the do your work with an actual brush and blend into ink or watercolors, it's a great tool. Just test for potential flooding first.