Final answer:
The fiduciary responsible for managing the assets in a trust is called a trustee, who must act in the best interests of the beneficiaries and according to the trust's terms, especially once it becomes irrevocable.
Step-by-step explanation:
A fiduciary appointed to manage assets in a trust is known as a trustee. This individual has the duty of managing the trust's assets in the best interests of the beneficiaries, and in accordance with the terms of the trust document.
Trustees are often selected by the person who creates the trust (the grantor) and can be heirs, attorneys, or other trusted individuals or entities.
Once the trust becomes irrevocable, such as upon the death of the grantor, the trustee must act in accordance with the trust's instructions without alteration.