Idiom

a knight in shining armour / armor

Meaning: If someone is a knight in shining armour, they help you when you are in a difficult situation.

For example:

• Jason was my knight in shining armour. He brought food and newspapers every day until I got better.
  
• People thought the new president was a knight in shining armour, but they soon found out he was just another politician.
  

Note: "Armour" is British spelling and "armor" is American spelling. Both spellings are used in Australia.