Humaaans: Mix-&-Match illustration library
Customizable 2D illustrations of people. These images are licensed CC0, so they're free to use.
Free to Use Clip Art Images & Vector Illustrations | ManyPixels
Free illustrations. This isn't a huge library, but you can download them as SVGs and customize the images. There are some images of people that could potentially be used for characters in a scenario with some customization.
unDraw - Open source illustrations for any idea
Image library that you can easily customize. These characters might work in some scenarios, but they don't have facial expressions. Available in SVG and PNG.