Penumbra tends to show a 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 to which she gets no response from her. Penumbra eventually manages to contact Della, explaining that with Scrooge McDuck's defense satellites down due to Louie's attempts to stop Bombie, Lunaris has launched the invasion on Earth.
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 after Lunaris' defeat, Penumbra encounters Launchpad, who becomes smitten with her.
The episode "They Put a Moonlander on the Earth!" focuses on Penumbra who is having a hard time adapting to her life on Earth ever since after Lunaris' defeat during the events of Season 2. In it, Webby encounters her outside the McDucks' house to which Penumbra explains to her that she is busy repairing her ship so that she can return to the Moon.
Della later encounters her repairing her ship, explaining to her that she hadn't experience of what life is like on Earth, to which Penumbra denies, explaining to her that she went out for coffee during her date with Launchpad McQuack. As Dewey and Webby arrive, Della tells them to take Penumbra to have some fun across Duckburg to which Penumbra explains that she has a meeting with Gibbous and Zenith who are talking with her about their fun time on Earth. Penumbra, however, changes her mind after being homesick from the Moon. While on the limo on their way to the Duckburg Pier.
Later upon arriving at the pier, Webby and Dewey give Penumbra a montage of having fun on the pier to which she still ends up not getting used to having fun on Earth until later at the Hamburger Hippo restaurant after getting a bit used to having fun with Webby on Earth by trying some of the restaurant's burgers to which the moment suddenly becomes interrupted when she ends up getting brain freeze after eating half of the sundae to which she briefly goes on a rampage. Noticing of what Penumbra is doing, two guards and the safety inspector try to arrest Penumbra, believing that she is a threat to the Earthlings to which Penumbra manages to escape from them.
Furthermore, Penumbra ends up noticing Webby and Dewey trapped on Flintheart Glomgold's attraction the Glomwheel on one of the attraction's gondolas to which she asks Gibbous to help her save the two. Penumbra manages to rescue Webby and Dewey who are stuck on Glomgold's attraction to which Penumbra starts getting used to having fun on Earth after feeling uncomfortable at first. After Glomgold is fined for safety violations and damage to the pier, Penumbra tells Launchpad that he would be a perfect ally for the battlefield. The next day during another conversation between Gibbous and Zenith, Webby asks Penumbra to know if she could stay on the Earth to which she explains that she could unless Webby could take care of herself. Launchpad later arrives to which after talking with the two Moonlanders talking with Della's family, just as Penumbra boards Launchpad's limo after talking with Gibbous and Zenith about herself getting used to fun moments on Earth.
- Her name comes from the term for part of a shadow between its darkest part, the umbra, and the light around the shadow.
- In "They Put a Moonlander on the Earth!", Penumbra tells Launchpad McQuack that the reason she is not interested in him was because "[she doesn't] want to date an Earth... male". Shortly after the episode was released, writer Sam King confirmed that she is a lesbian. This makes Penumbra the second major animated Disney character to be confirmed as lesbian, after The Owl House's Amity Blight.
- Since Penumbra debuted earlier than Amity, this also technically makes her Disney's first major lesbian animated character, although Amity's sexual orientation was confirmed before Penumbra's (not to mention Amity was already confirmed to be part of the LGBTQ+ community before she was confirmed to be lesbian).
- Curiously, The Owl House creator/executive producer Dana Terrace previously served as a director on DuckTales during its first season.