A Congress member acts a trustee by acting in the best interest of the people he or she represents. A trustee is a person who is legally appointed to manage someone's affairs when that person is physically or mentally unable to do so. A Congress member is a government official who is elected to represent the people within his district. Both people act in lieu or in loco of another.
The United States is a democratic republic. In a republic, officials are elected or appointed to act in the interest of citizens. In the United States, those officials are elected based on the majority vote of the people, thus the republic is democratic. Theoretically, the individual elected to represent a district in Congress is the candidate whose views most closely align with the majority of people in that district, which is what makes that individual the best Congressional representative. A trustee is legally appointed to handle another's affairs. Sometimes, the individual whose affairs the trustee assumes is selected by the individual. In certain situations, such as dire medical situations in which the person is incapable of making decisions or when the individual in question is extremely young, a trustee is appointed. Whether appointed or chosen, however, the trustee is, theoretically, an individual who is capable of making decisions in place of the individual whose affairs he is managing as if he was that individual.