Actually it would be either the holder, a non holder in possession or their agent/servicer.
U.C.C. - ARTICLE 3 - NEGOTIABLE INSTRUMENTS
..PART 3. ENFORCEMENT OF INSTRUMENTS
§ 3-301. PERSON ENTITLED TO ENFORCE INSTRUMENT.
"Person entitled to enforce" an instrument means (i) the holder of the instrument, (ii) a nonholder in possession of the instrument who has the rights of a holder, or (iii) a person not in possession of the instrument who is entitled to enforce the instrument pursuant to Section 3-309 or 3-418(d). A person may be a person entitled to enforce the instrument even though the person is not the owner of the instrument or is in wrongful possession of the instrument.