An ahoge (top of the head).
Ahoge (アホ毛, アホげ), literally foolish hair, is a visual cue common to Japanese anime and manga. Consisting of a single cute, often long, lock of hair sticking out from the top of the head, it is most often used to identify foolish, bumbling or carefree characters. In the west, Alfalfa from The Little Rascals would be the most identifiable character with what could be considered an ahoge. Ahoge have appeared on numerous characters but are usually limited to those appearing in works with comedic content and then mostly on female characters. Male characters with ahoge are more rare but are not unknown.
Ahoge are a naturally occurring trait of the hair and are not created with the aid of styling tools. They also appear only near the top of the head, but can be anywhere on top, front or back. The look of ahoge can also vary widely, from long and thin to short and thick.
Ahoge are especially common in the Danganronpa series, acting as the main definable feature for each game's protagonist, and are also a key feature of Servant x Service.
Hasebe tugs on Lucy's ahoge in Servant x Service