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Kosumi FS
A model in Kyuseido's fountain pen lineup. Specs below reflect the typical configuration across Kyuseido's core range.
A solid mid-range option once someone has decided fountain pens are a habit worth investing a bit more into.
Specs
About Kyuseido
Kyuseido is a Tokyo-based pen brand founded in 2022 by CY, who had earlier started the retailer Tokyo Station Pens and co-hosts the Tokyo Inklings podcast. The brand combines traditional Japanese craftsmanship such as urushi work with modern engineering, and is best known for the Kakari FS fountain pen with a pump piston filling system developed with Ben Walsh of Gravitas, as well as for nib grinding services.
How to fill the Kyuseido Kosumi FS
Fill it two ways: snap in a pre-filled international cartridge for convenience, or use a converter (a small plunger or twist mechanism that installs the same way a cartridge does) to draw ink from any bottle. The converter route opens up the entire world of bottled ink, which is where most of the hobby's color variety lives.
Choosing a nib size
Nib size determines line width, not writing pressure: fountain pens are meant to glide, never press. Extra-fine (EF) suits small, dense handwriting and thin paper; fine (F) is the most broadly recommended default; medium (M) shows more ink character and shading; broad (B) and stub grinds favor bold, expressive strokes and calligraphy-adjacent writing.
Keeping it clean
A quick flush with cool water every time you switch ink colors, plus a full clean every 4 to 6 weeks of regular use, is enough to keep the nib and feed performing well. Inktend's cleaning reminder is tuned to a 21-day baseline and adjusts naturally to how often you actually refill.
Pairing inks with the Kosumi FS
Because refills are quick with a cartridge or converter, this is a good pen for rotating through several colors rather than committing to one; sample vials work especially well here.
Frequently asked questions
More from Kyuseido
Specs reflect the typical configuration across Kyuseido's core lineup rather than this exact model. Check current listings before buying.