Pietro is Penny's father and creator. Penny tells Ruby Rose that she was created by her father in "Painting the Town..." with help from James Ironwood. She appears to love her father very much, calling him "very sweet".
In ”The Greatest Kingdom”, Pietro reveals that his daughter had told him all about her time with Team RWBY, and explains to Ruby and the others that he rebuilt her after her core was recovered in Amity Arena Colosseum after making its way back to Atlas after the Fall of Beacon.
In "Worst Case Scenario" it was revealed that Pietro created Penny as the next breakthrough in defense technology, since he wanted a protector with a soul. Therefore to create Penny's Aura, he gave her part of his own, and to rebuild her takes more from him. Fearing that if the people would have Penny destroyed, it would cause the part of his Aura in Penny to be lost.
Penny first takes a liking to Ruby in "The Stray", when Ruby calls her "friend". Excited and eager to have a friend, she asks Ruby for confirmation on their relationship being a friendship, to which Ruby replies "Yeah, sure. Why not?" Although quick to call her a friend, Ruby does seem put off by Penny's excitement over it and even asks Weiss Schnee if that was what it was like when they met.
Penny admits to Ruby that she does not have a lot of friends, and thus greatly treasures their friendship. She is also extremely protective of Ruby, going as far as to jump into a fight in her defense when she was injured in "Black and White", and as seen during "A Minor Hiccup", where she pushes Ruby out the way of an incoming truck before stopping it in its tracks, visibly shocking Ruby.
Penny considers Ruby to be a valuable friend, placing a high amount of trust in her and going as far to tell her that she is not a real girl. In "Painting the Town..." after revealing such news to Ruby, Penny was surprised by how the news was taken by Ruby, who stated that she did not care, for she was still real and carried both a heart and soul. Penny was overjoyed at the fact that her friend still accepted her as a person, and wrapped her in a tight hug, saying she was the best friend anyone could have. Also, in Never Miss a Beat, she tackled Ruby in a hug. Likewise, Ruby was devastated over Penny's destruction in "PvP", dropping to her knees and sobbing.
It appears that Penny is exaggerated for the reason that she told Ruby: she does not have a lot of friends. This is possibly due to her being isolated, given that she has had no time to socialize.
In “The Greatest Kingdom“, after Penny took down the remaining Sabrys, she express great happiness to see Ruby in Mantle. She uses her turbo boosts to speed up to get close to Ruby and give her a big hug. After she explains to Ruby and the others how she is alive, she hopes she reconnected with Ruby again until another disaster in Mantle interrupted their reunion shortly.
Weiss runs right into Penny while chasing down Sun Wukong. She expresses skepticism when Penny says she is going to be in the combat tournament.
Both she and Yang Xiao Long run off after Penny arrives to talk to them while they are looking for Blake Belladonna, apparently not wanting anything to do with her. It appears she has an admiration for Weiss' hair on the second day they meet.
However, later in the series it appears that Weiss has made a small connection with Penny, feeling sad about her death.
Due to Penny's odd and robotic behavior, Yang finds her bizarre to the point of neglecting the pleasantry of a simple introduction until Blake Belladonna reminds her. Both she and Weiss quickly run off when Penny shows up to talk to them while they are looking for Blake, indicating a lack of interest in associating with her.
In "End of the Beginning", Yang was remorseful towards Penny after her identity was revealed as a robot when she was destroyed by Pyrrha.
Ciel is part of Penny's team in the Vytal Festival Tournament. After their team progressed past the team stage of the tournament, she and Penny were selected to represent them in the doubles round. Penny states that Ciel was told to be Penny's friend and as such is more of a bodyguard to her. Ciel keeps track of all the places Penny is and for how long she is there. Because she does not see Ciel as a true friend like Ruby, Penny has not told Ciel that she is a synthetic being.
When Penny and Pyrrha are about to compete, Penny expresses pleasure in finally meeting her. Those who are aware of both Pyrrha's Semblance and Penny's true nature know that the two of them competing against each other has the potential for disaster. Subjected to a hallucination by Emerald Sustrai, Pyrrha uses her polarity with extra strength to deflect what she sees as an abundance of swords about to strike her. Penny is destroyed as a result. Pyrrha is left horrified and devastated after seeing what she has done.
While they weren't shown interacting at Beacon, Nora was one of the many of the group to be surprised that Penny is still alive at Mantle, saying that she had a lot of questions upon seeing the android, though she also smiled when Penny reunited with Ruby.
In "A New Approach" Penny guided Nora along with their other friends into discovering Atlas academy and their dorm though her enthusiasm and long visit of the academy eventually caused Nora and the others to fall out of exhaustion.
In "Pomp and Circumstance" Nora joins in when Penny has a moment of laugh with Yang after being surprised by her detachable robotic arm, implying that Nora enjoys Penny's company.
Cinder discovers through her computer virus the truth about Penny's nature. She shares this knowledge with Emerald and Mercury, and presumably comes up with the plan for her destruction.
In "With Friends Like These", when Penny encounters Cinder in person, she seems to have personal grudge towards her.
Mercury prevents Ruby from attempting to stop the match between Pyrrha and Penny, taunting her about it.
Penny first sees Roman Torchwick when she and Ruby arrive on the docks in "Black and White." At first confused by his casual greeting into wondering whether he is Ruby's friend, she immediately becomes focused after he shoots Ruby. She jumps into the fight, quickly neutralizing Roman's men, which makes him decide to escape. Her fighting style prompts Roman into commenting that "these kids just keep getting weirder."
At the end of "Black and White", Penny is collected by the driver of a black car. The driver reproves Penny for taking risks by going out alone. This individual is unidentified but Penny refers to him as "Sir" in a subdued tone, implying that he has a strong position of authority over her.
In RWBY: The Official Manga, when the driver asks her how was her pleasant walk and Penny replies she was fine with a sadden smile.
Penny is seen standing with James and a group of other Atlesian military personnel in the Volume 2 Opening. He is revealed to have assisted and sponsored her father in her creation, with the hopes that she will one day be able to save the world. Penny is also seen following Ironwood around in "It's Brawl in the Family."
In "With Friends Like These", after Penny along with Winter heard Ruby's warning about Ironwood's plan to abandon Mantle, she seems to be very disturbed and shocked about his plan.
After Penny becomes the new Winter Maiden, she decides to abandon Ironwood and sides with Ruby and her friends.
Penny and Winter share a professional yet good relationship with each other along with both of them are some of the people Ironwood told the secrets of the Maiden powers, the Relics and Salem.
Penny confronts Fria and asks her if she’s alright until Fria calms down. Penny also accepts the power of the Winter Maiden as Fria uses the last of her strength to transfer her powers to her. Afterwards, Penny grieves over Fria’s death.
Although she runs away from them at first, she does not appear to have a poor relationship with the Atlas soldiers. When Ruby expresses concern about her going with them, she says that "They're not bad people". She is shown respecting them and following their orders. She is also shown to be doing "the robot" with one of the soldiers in "Dance Dance Infiltration."