Is imagining robin hood talking to you, even when you try not to, a sign of a brain tumor?

Very unlikely a. Serious physical issue, such as a tumor. An obsessional delusion is a neurotic psychiatric issue, that might involve talk or medication depending on how much this interferes with your day, your sleep, or your happiness.
Not necessarily. This could be related to mental health issues or other conditions including malignancy. I would suggest that you seek an expert opinion on this situation sooner rather than later. See your primary physician immediately for further evaluation and referral to an appropriate specialist, if needed.