nos

nos is the first-person plural, and it is short work: nominative and accusative are both nos, dative and ablative are both nobis. The genitive splits in two, and the split matters. nostri is the objective genitive, "of us" as the object of a feeling (memor nostri, mindful of us); nostrum is the partitive one, "of us" out of a group (unus nostrum, one of us).

With cum the ablative takes the preposition as a suffix: nobiscum, never cum nobis, exactly as with mecum and tecum. And since the verb ending already says "we", a nominative nos is there for emphasis or contrast rather than information.