I was wondering what it represents? I've been researching and I could not find any information or reason regarding what they are exactly or why they are placed underneath the eyes. This, however, seems to be an anime-only thing and, possibly, an artistic style or preference, unique to the one who drew them. Below is the cover for the first light volume, which do not have those lines.
Is it supposed to be like having dark circles under the eyes like some people in real life or something else? No, they are not dark circles under the eyes. Dark circles under the eyes are different, and can be seen clearly. An example is from Watamote.
They are also not underlashes as both characters also have them and they are drawn separately. So far, I think the only way we'll know is if the one who drew them reveals the reason and purpose for placing those lines.