I would agree with the comments that the individual is a trustee and should be described as one. Their appointment should have been minuted, so no matter what they want to be called, they are a trustee. They can resign of course. I am not sure if the council is then under a duty to provide a replacement. I note that some clerks dissuade councillors from becoming charity trustees, on the basis that the council may, but is not required to, appoint such trustees (and no councilllors may want to be trustees, and you have to willingly accept the 'gift'). The declaration of interest is also an issue which weights on trustees.