from the character traits file:

GensOctavia character has 50% chance to be popularis

20% when you acquire Priest of Ceres
20% when worshipper of Ceres build the Temple of Ceres
2% when worshipper of Ceres end in city with the Temple of Ceres
5% every turn for patrician character
15% every turn for plebeian character

there is also 5% hance that character with high influence and politic skill becomes popularis
xxxx

So how to increase chances for this trait? Accept plebeian characters into your family and build temples of Ceres.