Quite possibly the best 0.5mm pen I have ever used, and I've used them all! 
 
Super smooth flowing, black ink doesn't skip and dries instantaneously. 
 
Zento Flow is an upgraded version of the regular Zento pens, featuring a cool anodized aluminum grip that adds weight and style to the pen. The refills are interchangeable with those used in the original Zento pens.
 
0.5 mm; black ink. Refillable with Uni Zento cartridges. Made in Japan.
 
Specs
Tip: 0.5 mm; black water-based pigment ink — uni's low-friction ZENTO formula resists feathering and bleed-through
Refillable (interchangeable with standard ZENTO refills); made in Japan