Arcus

Iris can change her appearance into her Roman counterpart of Arcus. As Iris is a free spirit who only attacks in self defense, it is unknown if she becomes more militaristic and warlike in her Roman aspect.

The Son of Neptune
Arcus appears in as her Greek counterpart of Iris even though she meets two Roman demigods; Hazel and Frank.

Trivia

 * Roman demigods do not use Iris Messaging, as by the time that Rome conquered Greece, they used more advanced methods such as "giant eagles" or their road system.