Any party may appoint a representative (whether a legal representative or not) (rule 11(1)).
Also, a party may be ‘accompanied by another person … who … with the permission of the Tribunal, may act as a representative or otherwise assist in presenting the party’s case at the hearing’ (rule 11(5)).
If a party has not appointed a representative, the Tribunal may appoint a legal representative where (rule 11(7)):
(a) the patient has stated that they do not wish to conduct their own case or that they wish to be represented; or
(b) the patient lacks the capacity to appoint a representative but the Tribunal believes that it is in the patient’s best interests for the patient to be represented.
The Tribunal panel must keep the patient’s capacity in this regard under review during the hearing, and an appointment may be made for a patient with fluctuating capacity who had previously appointed his own representative (PI v West London Mental Health NHS TrustM,  MHLO 8).
A personal welfare deputy cannot appoint himself (or anyone else) as a representative unless the order appointing him expressly provides for this (AMA v Greater Manchester West MH NHSFT  UKUT 36).
Note that the Tribunal cannot appoint a non-legal representative. ‘Legal representative’ is defined in rule 1 as:
a person who, for the purposes of the Legal Services Act 2007, is an authorised person in relation to an activity which constitutes the exercise of a right of audience or the conduct of litigation within the meaning of that Act
In practice, this means solicitors, barristers and legal executives. The secretariat will only appoint Law Society accreditation scheme members.
(Assessing the client’s capacity is considered in detail elsewhere.)