Nishizawa Kazuki is the younger brother of Ayumu and a friend of Wataru. A year ago, Kazuki met Nagi who was trying to get a balloon which was stuck in a tree and fell in love with her at first sight. When he met her again in Wataru's store he invited her to a walk and finally confessed his love to her.

During their "date" she told she already loves someone and after Hayate saving her from falling in a lake Kazuki realized it was him she loves.
(Source: Wikipedia)