Sera is a type five Invid. Until Sera and Ariel rebelled, the Regis had considered the human form to be the ultimate life-form. Now she's not so sure.
Sera and Corg were created as the first of the superior Invid beings, but the human form and the emotions that come with it makes Sera see too many similarities between humans and herself to stay enemies. Her emotions are further taxed when she becomes irrationally attracted to Lancer (it's love at first sight).
Ultimately, Sera argues against the Invids tactics and pleads with the Regis to give up the war and try to live in harmony with humans. The argument is enough to make the Regis question her motives and is partly responsible for the Invid's brief departure from Earth.
NOTE: As a non-player character, Sera is found in the company of her love, Lancer. Although she would rather reach a peaceful settlement with the Invid, she is ready and willing to fight to protect her adopted planet and humankind.
Lancer has taught her to operate Earth weapons and vehicles, but she is most adept with her Royal Command Battloid. She is familiar with hive layout, operation, and strategies. She is branded a traitor by the Regis and known to ALL the Earth's Invid (empathically). She will not be able to command or bluff these Invid troops.
The humans don't know quite what to think of her, except to view her with suspicion. It is only Lancer's reputation that makes the freedom fighters accept her at all.