"That, dear girl, is a lie."Lies are a small, adorable mashmallowy creature with no facial features except for eyes, which live in RGB's world. This makes them a surprising favorite among fans. We have only definitively met one of these Lies over the course of the comic, where Hero found one near the Pool of Tears in chapter 2, and then let it go again to wander back into the water. We do not know whose lie it was, if these creatures are summoned by untruths.
The design of the creatures is almost definitely a reference to the term "little white lies"