In HDRP, only one directional light can cast shadow at a time. This can be undesirable when we need lighting that differs from the scene's main directional light, such as brightening a character's face or adding idealized shadows.
The Box Light
offers a practical workaround, and Unity Toon Shader supports it.
Drag the slider to compare the images.
Box light applied to a character's face. Note that editing the angle of the box changes the shadows falling on the face, but not on the body and the ground.