I just imagined how Felix would look with a mustache and it was good in my mind. So I decided to draw it out to see if my artstyle can capture the image. If you want to see him with a thicker mustache, here is the link: https://www.colorslive.com/details/5130249 The eyes are darker and the hair shadings are lighter because this was before the revision.

Fire Emblem (c) Nintendo