38 CFR §19.50

1. (a)Persons authorized. A Notice of Disagreement and/or a Substantive Appeal may be filed by a claimant personally, or by his or her representative if a proper Power of Attorney or declaration of representation, as applicable, is on record or accompanies such Notice of Disagreement or Substantive Appeal.
2. (b)Claimant rated incompetent by Department of Veterans Affairs or under disability and unable to file. If an appeal is not filed by a person listed in paragraph (a) of this section, and the claimant is rated incompetent by the Department of Veterans Affairs or has a physical, mental, or legal disability which prevents the filing of an appeal on his or her own behalf, a Notice of Disagreement and a Substantive Appeal may be filed by a fiduciary appointed to manage the claimant's affairs by the Department of Veterans Affairs or a court, or by a person acting as next friend if the appointed fiduciary fails to take needed action or no fiduciary has been appointed.
3. (c)Claimant under disability and able to file. Notwithstanding the fact that a fiduciary may have been appointed for a claimant, an appeal filed by a claimant will be accepted.