Transparency is a vital part of healthy relationships, but not everyone is comfortable sharing their financial status, for a variety of reasons. Whether a character's motivation is noble, self-serving, or coming from a place of fear and distrust, no one likes being lied to, and a secret this big is bound to come out eventually, laying the foundation for inter-relational conflict down the road.

Financial Matters

BEING AN ANONYMOUS FINANCIAL DONOR, GAINING WEALTH THROUGH UNETHICAL MEANS

BECOMING WHAT ONE HATES, BEING JUDGED, BEING LABELED, BEING RESPONSIBLE FOR OTHERS, BEING TAKEN ADVANTAGE OF, BEING UNSAFE, BETRAYAL, COMPETITION, CONDITIONAL LOVE, CRITICISM, ISOLATION, LEADING, LOSING THE RESPECT OF OTHERS, REJECTION, RELATIONAL COMMITMENT, TRUSTING OTHERS

Having to live well below their means to maintain the façade
Always worrying that if people find out they're rich, it will change the way they view the character
Money taking up too much space in the character's mind and choices, making it too high a priority for them
Being reluctant to develop intimate relationships for fear the truth will be discovered
Becoming stingy (if they're hiding their wealth to keep others from accessing it)

Living frugally
Helping friends or family surreptitiously through anonymous donations
Avoiding high-end purchases
Hiding their wealth through secret accounts, lockboxes, or in a secret physical location (e.g. buried beneath the floorboards)
Pretending to go to work or have a job (if the character is independently wealthy)
Placing their wealth in the name of an intermediary, such as a family member or business partner
Using a lawyer or steward to manage their wealth, or channeling it through a business
Claiming to be struggling financially