Penny was originally Pietro's creation designed for a project approved by James Ironwood, as a means for a breakthrough in defense technology. He cared for Penny and came to see her as his own daughter.
In "The Greatest Kingdom", Pietro revealed that his daughter had told him all about her time with Team RWBY, explaining to Ruby and the others that he rebuilt her after her cores were recovered in Amity Arena Colosseum after making its way back to Atlas after the Fall of Beacon.
In "Worst Case Scenario" Pietro revealed how he created Penny by giving her part of his own Aura, and how rebuilding her took more from him. He feared that if the people had Penny destroyed after her framing, he would be unable to bring her back.
In "Amity", Pietro was strongly against Penny to push Amity Tower up because he didn't want to lose her again, but she managed to convince him. When Penny was getting hacked by Watts' virus, he became worried and told her to return back, but she was unable to go back. Pietro was later seen grieving over Penny after her communication was lost.
As Team RWBY are among the very first friends his daughter ever made, Pietro is shown to be very grateful to them. Upon meeting them in "The Greatest Kingdom", he explains how Penny spoke highly of her time with them in Vale.
During the events of Volume 4, Pietro was commissioned by Ironwood to build Yang a prosthetic cybernetic arm to replace the one she lost during the Fall of Beacon. Upon officially meeting Yang in "The Greatest Kingdom" he expressed surprise at Yang for having painted it.
In "Worst Case Scenario" Pietro reveals to Weiss and Ruby the truth regarding his and Penny's respective condition and was comforted in knowing that the two girls would protect his daughter no matter what.
In "With Friends Like These", he assisted Team RWBY by helping reactivate the communication on their Scrolls.
In "The Enemy of Trust", Pietro warmed up to Ruby and Weiss upon seeing them comfort Penny, clearly delighted to know how strong their friendships with his daughter was.
Ironwood was the one who helped him build Penny. In "The Greatest Kingdom", he seemed to be concerned for Ironwood's current psychological condition after the Fall of Beacon and took note of his current emotional disposition of paranoia towards not only the other Kingdoms but towards his fellow countrymen of Atlas.
In "Amity", he decided to help Ruby's Group finish Ironwood's original plan by getting Amity Communications Tower up and sending Remnant a message about Salem. While Ironwood saw the broadcast, he decided to hack Pietro's daughter Penny to bring her back to his side.
There has not been much interaction between them, but Pietro was the one who built Maria's prosthetic eyes after she lost her own. In "The Greatest Kingdom", when she visits Pietro at his clinic, he memory of her was fuzzy until she reminded him. It was then revealed that the two are close friends with each other, as seen where Maria assisted and cared for Pietro in his weakened state.
In "As Above, So Below", during the mass riot in Mantle when its heating grid suddenly shut down, Maria attempted to bring Pietro inside into his clinic when the riot began becoming increasingly violent and uncontrollable and later showed a look of horror and fear as Mantle citizens began brutalizing Atlesian Knight-200s and showed clear panic together with her as the Grimm alarms sounded.
Later in "Amity", Maria can be seen assisting Pietro in putting the finishing touches on Amity Communications Tower using a construction mech. Later Maria would attempt to give Pietro some advice on how to deal with Penny due to his overprotectiveness for her.
Later after Penny loses consciousness and fall into the atmosphere due to Watt's hacking attempt, Maria would try to comfort the concerned Pietro.
Not much interaction occurred between them on screen yet, but there was implication that he used to be close to Watts. In "Worst Case Scenario", he covered up Watts when he showed an old photo to Ruby, Weiss and Maria. It was hinted his relationship with Watts before the latter's apparent death was strained.
In "Gravity", Watts himself was bitter towards Pietro, for being chosen by Ironwood over him, calling Pietro a "fat imbecile" out of spite.