During the first cut scene it is introduced, it's barely larger than an Ogretail when a Vajra attacks it. After it killed the Vajra and devoured it's core, the Arius grows in size to match that of a Vajra.
This transformation also grants the Arius a cape similar to a Vajra, while a wing like appendage also extends on the back, mimicking that of a Dyaus Pita. The front legs gauntlet like exterior lengthens and on each upper hind leg the three stumps extend to a few feet length.
Born from the remnants of Nova, the Arius Nova is a fearsome Aragami, one with an acquired immunity to God Arc attacks thanks to the multitude of Bias factor strains it takes from other Aragami as it encounters them.
After defeating the entire first Unit in their first encounter, the God Eaters regroup and beat the Aragami in a short fight. However, the Aragami then reveals its immunity to their attacks, and attacks them by surprise as they think it's dead.
Afterwards, its continued devouring of other Aragami led it to leave its larval state and assume a more mature form.