noster

noster, nostra, nostrum is a first-and-second declension adjective of the -er type: the e survives only in the masculine nominative singular and drops everywhere else, giving nostri, nostro, nostrorum, nostris. It agrees with the thing owned, never with the owner.

Latin leaves possessives out far more readily than English does, so do not reach for noster unless the ownership is actually in question. The masculine plural nostri on its own is a noun in military prose, "our men, our side", and Caesar uses it constantly. Keep those forms apart from nostrum and nostri as genitives of the pronoun nos, which look identical: unus nostrum is "one of us", partitive, while the nostrum in hoc nostrum est is this adjective.