a preposition as well as an indicator for a person who is absent, of a singular number -- i.e. third person object prefix
Alonso de Molina:
c. es preposicion, y tambien es señal de persona que padece, de numero singular. Alonso de Molina, Vocabulario en lengua castellana y mexicana y mexicana y castellana, 1571, part 2, Nahuatl to Spanish, f. 10r. col. 2. Thanks to Joe Campbell for providing the transcription.