This review is by Kalina Wilson, who can also be found at geminica.com.
The Kuretake Fudegokochi gray brush pen is a water-soluble pen with a soft felt tip and middle-toned gray ink that seems especially suitable for use in drawing.
Luckily, the design of the pen keeps me from putting my hands in the ink, so it just stays where it is around the nib.
The ink is not only water soluble, it's very water soluble. Once the ink is down it can be washed out like watercolors, as long as it's used on sized paper such as watercolor or mixed media paper - or this vintage accounting ledger paper I've been trying out after meeting urban sketcher and illustrator Lapin.
(Actually, this drawing captures another urban sketcher trick as well; it was Liz Steel that clued me in to wearing an absorbent wristband as a convenient alternative to a watercolors rag. Mine is actually the top of an old cotton sock, and it works great.)
But back to the Fudegokochi! In the drawing of my hand, I used the Fudegokochi first to sketch and add shadow, then added black line and watercolors, washing away the gray lines in most places and sometimes adding more back in to darken. You can still see the initial gray shading in some places, and it's also influencing the tone of the watercolors in a way that I really enjoy.
My other favorite way to use this pen is to imply distance, using the gray to add buildings that are farther afield and a warmer tone in the foreground. That's what I did with this drawing that you already saw in the last review, pairing the gray pen with J. Herbin's Lie de Thé fountain pen ink. You can see that the water solubility again worked to my advantage.
In case you can't tell, I really love using this pen as a fast way to add soft gray to a drawing, and highly recommend it for this purpose.