The title of this article is conjectural!
Although this article is based on canonical information, the actual name of this subject is pure conjecture.
The Fourteenth's Supporter is an old man who kept the fourteenth's will in his house.
Very little is shown about his appearance. He is said to be an old man and we are shown his arm that is under perfusions.
He was given the testament of Nea that indicates the fourteenth was killed by the earl and his powers will be passed. He kept the device in his house until he was found by Lvellie who retrieved the will and presumably took the old man too. In the anime it is said he asked for protection from the Black Order as he was afraid he could be killed by the earl due to his connection with the fourteenth.Thus he collaborated with them and provided informations.