In Zabuza's second fight with Kakashi, Haku got between the two and sacrificed himself, becoming impaled by Kakashi's chidori. This was how Haku died.
Haku-I love you too, Chihiro.
This brave, and loving creature protects and bravely watches over young Chihiro, giving her back her identity and hope.
People often fall for his kindness, and affecton.
Although he keeps his emotions locked away, he is one of the most caring Dragons.