Shade in a dark background in a color like coal, slate gray or even black.
Streak gray chalk or watercolor in diagonal lines from the top of the picture to the bottom in the negative space around your anime character. Your streaks should travel in front of your character as well as behind to immerse him fully in the storm.
Place a "halo" (a thin layer of white or gray) around the head and shoulders of your character for a typically anime "hero" look. This also denotes a small splashing effect from the downpour without having to draw endless drops.
Choose a light or neutral background color such as white, beige, yellow or a pastel primary color.
Draw long diagonal dashes of rain in a primary color, such as red or blue, around your anime character. These dashes should be intermittent in length, but should all head in the same direction, such as from top right to bottom left.
Add point-of-impact drops on the head, shoulders and arms of your anime character. To do this, make four to five tiny diagonal dashes (in the same color) in an asterisk shape and repeat this in various locations on the body.