In the games:

Bel is one of the player character's childhood friends. She is clumsy and air-headed, but is a true friend. She often battles the player during his or her Pokemon journey.

In the anime:

Bel made her debut appearance in "Chillarmy is a Neat Freak!?," when she is sent by Professor Araragi to deliver a badge case to Ash. Unfortunately, Bel is a very clumsy girl and ends up knocking Ash and Dent into a nearby stream. Despite being older than Ash, she has only just started her journey due to her father not allowing her to become a trainer when she turned ten.

Her original name, "Bel," means "white" in Slavic, while her English name, "Bianca," means "white" in Italian.