There are many non-financial ways a character can become a secret ally or protector for someone. Examples include keeping tabs on an elderly neighbor, removing hate-graffiti from a local church building, or subtly intervening to remove a bad influence from a niece or nephew's life. While the object of their efforts would probably appreciate their help, the character may choose to remain anonymous if they want to avoid the spotlight or their involvement could cause conflict in some way (because they're supporting a controversial cause, for instance). 

Positive or Altruistic (primary), Hidden Identity

GIVING UP AN OPPORTUNITY FOR ALTRUISTIC REASONS, SAFEGUARDING SOMEONE ELSE'S SECRET

BEING ATTACKED, BEING JUDGED, BEING TAKEN ADVANTAGE OF, CONFLICT, CRITICISM, DISCRIMINATION, LEAVING NO LEGACY, LETTING OTHERS DOWN, LOSING ONE'S SOCIAL STANDING, LOSING THE RESPECT OF OTHERS, REJECTION

Having less free time because they're devoting a portion of it to helping someone else
Not being able to share what they're doing with people who might align with them, thus missing out on potentially helpful resources and expertise
Not receiving recognition for their work
Hating having to lie to loved ones (even if the character's motives are good)

Feigning surprise at the news of the task being done
Lying convincingly when the recipient asks the character if they're behind the kind act
Doing their work when no one is around to witness it
Wearing a disguise (sunglasses, a hat, etc.) when they go out to help
Never mentioning the cause or person they're helping
Using cash for any related expenses
Parking far away so no one will recognize their car at the site
Devising a regular errand or hobby that dovetails with the act of helping so it can be used as an alibi