A soft lining brush, the inside is more harsh while the outside of it is more like an airbrush, making it appear soft. It's nice for quick doodles!
Edit "2 - brush size" to make the airbrush effect more or less extreme,
I recommend editing the settings to your liking! Applying "Blend tips with darken" makes the center tip more harsh, while having it off (default) gives it a more soft feeling.