Sugar is optional in the standard French crêpe recipe on Wikipedia.
The basic batter is built from flour, eggs, milk, salt, and butter. Sugar is left out when the crêpe is intended for savoury fillings, but it can be added for sweet crêpes or dessert preparations. This flexibility is why sugar is not treated as essential to the standard batter.
A crêpe differs from a leavened pancake because its batter is liquid and spreads into a very thin layer. It is usually cooked quickly—about 20–30 seconds per side—in a hot, greased pan. The batter’s thinness, rather than sugar, is the defining feature.
A common mix-up is assuming that every crêpe must be sweet. French crêpes may be filled with fruit, chocolate, or jam, while savoury versions can contain cheese, ham, eggs, vegetables, or meat. Buckwheat galettes are another savoury tradition, especially associated with Brittany, but they are distinct from the wheat-flour batter described here.