The Cocoiro pen by Japanese brand Kuretake is small and ergonomic. It comes with a replaceable ink cartridge attached to an extra fine nib. There are a variety of pen colours and ink colours to choose from.
Each cartridge comes with a lid, so you can switch between different coloured nibs without waiting for the cartridge to run out. The base of the cartridge extends out of the bottom of the pen so you can check the ink level and colour.
The extra fine nib is flexible, slightly firm and great for drawing and lettering with varied widths.