Chinese laborers started settling on Guam in larger than usual numbers around the year 1858. That year alone saw the arrival of Chinese men named Unpingco, Losongco, Tyquiengco and perhaps that same year, or maybe soon after, Limtiaco.
By the 1880s, there were between 15 and 20 Chinese men living on Guam, maybe a third of them married to Chamorro women and baptized into the Catholic religion.
In a document from the year 1884, Rosauro Unpingco is described as the "current head of the Chinese residents in this island." "Cabeza actual de los chinos residentes en esta isla."
The Spanish colonial government system including neighborhood heads called cabezas de barangay. A barangay was a neighborhood, and its head was the cabeza (Spanish for "head").
As far as I know, there never was a formal office for a cabeza for the Chinese residents of Guam. At least I've never seen such a title on any official list of titles or positions in the island government during Spanish times.
So, I suspect that Rosauro's title was an informal, unofficial but practical one. He would act as the liaison between the Spanish government (its Spanish officials and Chamorro clerks) and the dozen or so Chinese residents on Guam. He would relay to the Chinese community any news, announcements or requests from the Spanish government, among other duties. His position would have been appointed, not elected (there were no elected offices in the Marianas at all under the Spanish).
Once again, don't be too concerned about the way Rosauro's last name is spelled. Here it is Ung Pinco. "Ung" would have been his Chinese last name. It is often rendered Ng today in his variety of Chinese (Fujian). In the Spanish records, Unpingco is spelled in three or four different ways.