Responsa על שבועות 61:8

a) Q. Is a person who reached his majority, permitted to appoint a legal representative to argue his case in court? The Palestinian Talmud (Sanh. 2, 1) definitely implies that a high priest is permitted to appoint such a representative.
A. The statement in the Palestinian Talmud does not necessarily imply that an ordinary person is permitted to appoint a legal representative. It is possible that only a high priest is permitted to appoint such a representative, in order to save him the humiliation of appearing before a court having less authority than himself; but an ordinary person may not be permitted to make the appointment. In any case, all are agreed that a levir is not permitted to appoint a legal representative when his brother's widow demands that he fulfill the positive command (of God) to perform the ceremony of halitzah or to marry her. Moreover, a person commits a wrong when he consents to become a legal representative to a levir, thus helping to postpone the fulfillment of this positive command, and injuring the interests of the widow.
b) Q. Is a legal representative permitted to present before the court claims and arguments that will improve his client's case but which he knows to be false?
A. The plaintiff, or defendant, himself is not permitted to lie in order to improve his case in court; a legal representative, therefore, is surely not permitted to lie.
SOURCES: Cr. 246; L. 126, 127; Mord. B. M. 276; cf. P. 519; Mordecai Hagadol, p. 270a.

b) Q. Is a legal representative permitted to present before the court claims and arguments that will improve his client's case but which he knows to be false?
A. The plaintiff, or defendant, himself is not permitted to lie in order to improve his case in court; a legal representative, therefore, is surely not permitted to lie.
SOURCES: Cr. 246; L. 126, 127; Mord. B. M. 276; cf. P. 519; Mordecai Hagadol, p. 270a.