Penumbra tends to show cold attitude and distrust towards outsiders as she showed little to no respect for Della when she discovered her while being attacked by a Moon Mite, and introducing her to their hidden city. Even when Della hugged her as a way of thanking them for leading her to fuel needed for her rocket, she resisted with all her might. She has explained that in the past she used many of her personal weapons including her childhood training spear to defend her home from unwanted pests, so it can be assumed any other species she's met that isn't of her own kind is her way of considering them as threats.
Penumbra often chooses her actions for the sake of her people and their safety. This is the reason why she continues to be keeping a close eye on Della during her time on the moon, believing she was going to put the other Moonlanders in danger. But she is also shown to be considerate towards others, as when Della had fixed her spear she broke and explained her true motivations for fixing the rocket, Penumbra began to warm up to her. This is proven further when as Della was about to leave, she gave one final hug to Penumbra as a farewell, and that time Penumbra didn't resist.
Penumbra's consideration for her people is also conflicted at times, as when Lunaris revealed his intent to use the rocket technology Della left behind to launch attacks on Earth, Penumbra was questionable about the idea, believing he could be putting their people in serious danger for such an action.
Penumbra is a lavender-skinned humanoid alien. Her eyes are a dark purple with white pupils, and her hair is white in a small pixie cut. Her golden armor consists of a helmet with a flat headpiece, a tunic with a green circle at the center, twin gauntlets, and twin boots.
Penumbra first appeared in "What Ever Happened to Della Duck?!", in which she and General Lunaris have been hunting down a moon mite for years. They encounter Della Duck, who has been fending off the mite for a decade while trying to fix her ship, the Spear of Selene. Though Penumbra desires to kill the mite, Della discovers that the mite merely has a baby and earns its trust peacefully. In seeming gratitude, Lunaris offers Della shelter and all the gold she can use to repair her ship at their city, Tranquility.
By "The Golden Spear!", Della has nearly finished repairs on her ship while staying at Penumbra's house. Penumbra spends the episode believing Della to be secretly an enemy of the moon, especially since Della frequently regales the Moonlanders of tales about Earth. In a desperate attempt to stop Della from enslaving her people, Penumbra activates the completed rocket, forcing Della to leave. However, Lunaris reveals his true colors and begins an invasion of Earth, causing Penumbra to realize how much she appreciates Della.
Sometime later, Penumbra encounters Della's brother Donald in "What Ever Happened to Donald Duck?!", who had crash-landed on the moon after discovering Della's repaired rocket. Though Lunaris declares Donald an enemy of the moon and promotes Penumbra to the rank of captain, she assists Donald in escaping captivity, eventually stumbling upon Lunaris's secret base. There, she learns that Lunaris had been analyzing Earth even before Della's arrival. Lunaris arrives and battles Donald, but Donald makes a getaway in a prototype rocket that nobody could supposedly survive. As Donald escapes, Penumbra declares him the bravest man of two worlds.
Throughout "The Richest Duck in the World!", Della attempts to contact Penumbra using cameras and cable. Penumbra eventually manages to contact Della, explaining that with Scrooge McDuck's defense satellites down, Lunaris has launched the invasion on Earth.
A week later, in "Moonvasion!", Penumbra finally arrives just as Lunaris is about to finish off Scrooge, Della, Donald, Webby, and Huey, Dewey, and Louie in space. Penumbra seemingly sacrifices herself to stop Lunaris, but lands on Della's window. Back on Earth, Penumbra encounters Launchpad, who becomes smitten with her.
- Her name comes from the term for part of a shadow between its darkest part, the umbra, and the light around the shadow.