V-아/어 주다
Add 주다 after an action to show that someone does it for another person, often with a kind or helpful nuance. In the story, 불러 주었다 means she sang a song for him. You can reuse this pattern in sentences like '친구가 사진을 찍어 주었다.'
그녀는 특별한 노래를 불러 주었다.
She sang a special song for him.
