The isolation has made Seri a fascinating language. It is comprised of 18 consonants and eight vowels, and it has a very large lexicon. Kinship, for example, has over 50 primary terms, making it one of the most extensive lexicons in any language in the world.
Another complexity is the use of plurals. Explained by Stephen A. Marlett in the Journal of the Southwest:
“Unlike languages that either do not have any indicator of number (some Zapotec languages in Oaxaca, for example), or just add the suffix –m to the noun (as some neighboring Uto-Aztecan languages in Sonora, such as Yaqui), or usually add –s (like English), Seri flourishes here. In fact, every noun and verb has to have its plural listed in the dictionary because one simply has to learn it.
In Seri, one could ask for a couple of dozen words at random and never see a common way for plurals to be formed. Verbs are similar in that they have different forms depending on whether the action was done by one person or more than one, and whether the action was repeated or not repeated (roughly speaking). And these forms display about the same kind of complexity as the nouns.”