The Cocoiro pen by Japanese brand Kuretake is small and ergonomic. It comes with a choice of pen colour, ink colour (with the extra fine nib) and nib type. The nib is attached to a replaceable ink cartridge and comes with a lid, so you can switch between brush or sign nib 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 brush nib comes loaded with black ink and is long and relatively soft, tapering to a fine point. It is comparable to a Pentel but slightly slimmer.