-ni.

Headword: 
-ni.
Principal English Translation: 

"that which is usually done," the customary action or habitual action, and, by extension, "one who typically does that thing"; e.g. cuicani = one who customarily sings; e.g. tecuani = he customarily eats people; or, this can also be a characteristic; and, added to verb in an "if" clause, such as "if I had done it"

Lockhart’s Nahuatl as Written: 

verbal suffix to indicate the habitual present and some optatives, but mainly it serves to form present agentives, as in tlahtoāni.
James Lockhart, Nahuatl as Written: Lessons in Older Written Nahuatl, with Copious Examples and Texts (Stanford: Stanford University Press and UCLA Latin American Studies, 2001), 227.

Attestations from sources in English: 

Mā xinēchcaquinî! = If only you (pl.) would listen to me!, Mācaīc ōnitlàtlacoāni! = Would that I had never sinned, I wish I had never sinned!
Michel Launey, An Introduction to Classical Nahuatl, translated and adapted by Christopher MacKay (Cambridge: Cambridge University Press, 2011), 166.

tēcuāni = people eater, wild beast
huel titlamatini = you're a very intelligent, wise person
ticochini = you are fond of sleeping
tlàtoāni = one who speaks, i.e. king, ruler
cuicani = singer
miquini = mortal
cualānini = irritable
Michel Launey, An Introduction to Classical Nahuatl, translated and adapted by Christopher MacKay (Cambridge: Cambridge University Press, 2011), 162, 163, 164.

Attestations from sources in Spanish: 

"El agente en los verbos. Existen varias terminaciones para indicar el agente en los verbos. Algunos tienen más de uno. Ejemplos: tecuani: comedor de gente." Este ejemplo tiene el posfijo -ni. Otros ejemplos:
tla-mati-ni = él que sabe algo, sabedor
cuica-ni = él que canta, cantor
tla-chihua-ni = hacedor
choca-ni = él que llora, llorador
chihuachocani = la que llora, la llorona
petlachihuani = hacedor de petates
calchihuani = hacedor de casas, arquitecto
teopanchihuani = hacedor de iglesias
tlatocani = enterrador de cosas
tetocani = enterrador de gente
tlahtohuani = él que habla cosas, gobernante
tequitini = trabajador
petlapacani = lavador de petates
tlaxcalchihuani = hacedor de tortillas
xochicualcohuani = comprador de fruta
tlaolmacani = dador de maíz
tlanamacani = vendedor
tlacohuani = comprador
iztactepoznamacani = vendedor de metal blanco
tecuani = comedor de gente, caníbal, jaguar
Fernando Horcasitas Pimentel, Náhuatl Práctico (1977), 26.