Who is Alvina from Love, Stargirl and what is their importance?

Alvina is the neighborhood bully. She is eleven-years-old, not afraid to fight boys, and as rough and tumble as a high school footballer. She wears clothes torn from her tussles, has wild flyaway hair, and a filthy mouth full of insults and threats. Despite all this, Stargirl is drawn to Alvina for her fearlessness.