Nino is the homeless girl who saved Kou Ichinomiya at the beginning of Arakawa Under the Bridge.
She lives underneath a bridge and claims she is a Venusian.
She was named by Chief because of her jacket that says "Year 2, Class 3" which is fully known as “Ni no San”, but she apparently acceptably shortened to just “Nino”.
Since Kou is in debt to her, he asks her what she wants and she says "(to) Experience love with me". Kou starts getting an asthma attack as a response to that and later states, "If this is what my savior commands then I am bound to deliver it even if it costs my life."