This article lists Rebecca Holiday's relationships with other characters in the series. Busy working as Providence's chief research officer, Holiday will get to meet others on few occasions.
Holiday's younger sister was an incurable spider-like EVO kept in containment at Providence. She was normally locked inside The Hole of the Petting Zoo. However, when Rex and Noah breached the Hole, she escaped and captured everyone in the tower, including Noah. Before Rex could destroy her, Holiday convinced Rex and Six to spare her sister, as long as the EVO didn't cause anymore trouble. It is implied that Holiday's sister was being used as blackmail material to get Dr. Holiday to work for Providence.
Her sister also saved her life when Skalamander cornered her near the Hole. Even though she had the chance to seriously injure Dr. Holiday, she instead went after Skalamander, implying that they share a deeper bond that Dr. Holiday may not even be aware of.
Later, Holiday gave her younger sister a teddy bear for her birthday. After Holiday attended a show presenting a cure for EVOs, she ran straight to Providence to get her sister into being cured as well. When Rex wasn't so sure about the idea, she strictly told him that he didn't understand. When coming to pick her sister up, Rex and Six found out that the "cure" was just a scam. Holiday, Rex, and Six went to find her sister without the approval of White Knight, but her mutations were enhanced. The man who ran the scam, Dr. Moses, then admitted being able to cure them while on the brink of being killed but escaped. Holiday made a dedicated attempt to the machine to cure, but there was only one required thing, Six's magnetic swords. Six jumped onto the machine, allowing his swords to make contact with it and getting knocked unconscious. Soon after Six awakened, her younger sister Beverly, who was successfully cured, ran to give Holiday a hug.
Rebecca also acts as an overprotective parent to Beverly rather than an older sister. Once, while she was at a Trendbenders concert, Holiday called both Beverly and Rex constantly to remind them to use earplugs and drink only bottled water. Meanwhile at the concert, Beverly was close to being a deputy for the Trendbenders along with Rex, Holiday had burst in on Beverly, taking her home right away.
- "Don't you do this to me. I will hate you forever if you–" — Holiday after thinking Agent Six is dead.
Even though they are virtually opposites, Six and Holiday seem to get along fine. However, they do argue sometimes about how to treat Rex. While Six generally thinks of him as a weapon in most cases, Holiday insists that Rex should be treated more like a human being. Lately, Six has shown much more compassion for Rex—perhaps a result of Holiday's advice.
Six and Holiday both respect each other and are willing to listen to each others' advice. Holiday's respect for Six might have had increased when Holiday's incurable EVO sister was taken down diligently under Six's orders. Six also defended Holiday's deal with White, which guaranteed the safety of her sister.
It is subtly implied that Holiday might have feelings for Six. For example, she attempted to hold his hand after she made an insensitive comment to him, but he walked away. She dressed up rather nicely for a party and asked where Six was when Rex arrived in his place, and later attempted to force a compliment out of Six. They are shown to be holding hands for a moment as corrupted nanites were regressing around them, thinking they were to be killed. When Six showed Holiday how Rex could be the "cure" she'd been searching for, she hugged him, though this may have just been because she was very grateful.
When a tested rabbit EVO was about to attack Six, Holiday fearfully pushed Six out of the way into a tree until the rabbit went back to eating. She looked somewhat embarrassed and said warily that he got "too close".  She also nearly gave up on curing her beloved sister until Six spoke some words of encouragement. After Six almost lost his life helping to cure Beverly, Holiday performed constant CPR. When he gained back consciousness, attempted to kiss him, but Rex and Beverly interrupted. Six, on the advice of Rex, later asked her out on a date, which she gladly accepted saying, that it was about time.
Following Six's memory loss, Holiday was a little unnerved by his more aggressive romantic overtures. However, when he was knocked into a coma, she planted a kiss on a status tube containing Six, so it appeared her feelings had not changed.
Later, Rebecca was very shocked when Six, who was actually John Scarecrow, kissed her. She had seemed so happy about it that she eventually confronted the real Six about it. When he stated he knew nothing about it, it angered Holiday deeply, making her think Six played with her emotions. Later, when Rebecca was on her own to find Scarecrow, she was taunted on her feelings on Six. Out of rage, she easily defeated him telling him in the end that it was for the kiss. This proved that it was evident that Holiday's feelings for Six are strong.
- "He deserves a celebration, he's changed everything." — Doctor Holiday on Rex
Unlike most of Providence, Holiday thinks of Rex as more than a weapon and gives him much encouragement. She is deeply concerned with his welfare, and will do anything it takes to keep him safe. For example, she was willing to use aggressive measures to save Rex when Breach captured him. Rex also jokingly said that "she worries too much about him". Dr. Holiday also showed that she was unwilling to leave the burning space station without Rex coming back alive and unharmed, even sacrificing a container of special nanite samples in order to stop Zag RS and save Rex. She also blasted Alpha for even trying to hurt Rex.
Despite the fact she's much older than Rex, Rex liked to flirt with Dr. Holiday a lot. While she was aware of his crush on her, she mostly ignored his flirtations. However, Holiday would occasionally use his crush to her own advantage, such as when she promised to have a dinner date with Rex if he'd bring her back EVO cell samples from the Bug Jar.
- "So blindly charging forward didn't work? I'm shocked."— Dr. Holiday, sarcastically, to White Knight.
Although Holiday works for White Knight, she does not always agree with his methods. She greatly dislikes White Knight's violent, miscalculated options when she thinks her ideas are clearly superior or when there's a better choice at hand. This once even caused her to snap and punch a computer screen, causing it to break.
White Knight also used Holiday's sister, Beverly, as blackmail material to get Dr. Holiday to continue working for Providence. With Beverly cured, it's likely that Dr. Holiday willingly continued to work for Providence for Rex's sake, and later on continued to work with White Knight's Providence Defect Group to counteract Black Knight and the Consortium.
- "If it means there are days when I don't find monkey hairs in my coffee, I roll with it."— Holiday to Rex about Bobo Haha.
When Bobo Haha and Holiday first met, Holiday was imprisoned with him by White Knight. They were both locked in a Providence cage that was used to contain EVOs. Lending a helping hand, Bobo picked the lock for Holiday and released her. Ever since then, the two have been good friends and partners. It's been implied by Holiday that Bobo tends to annoy her, which is why she never questions anything when Robo Bobo comes in the picture. She also dislikes it when Bobo makes sarcastic comments in serious situations and makes irresponsible choices, like borrowing her tools without permission.
Holiday evidently appreciates the fact that Noah is Rex's friend. She even allows the two to bond whenever they're together at Providence or whether or not they're together on missions. When Holiday had first found out that Noah was a spy, she tried to explain that sometimes people are forced to do what they don't and that it wasn't entirely his fault, somewhat altering Rex's view of him.
Holiday also cares deeply about Noah's safety. When Noah was first turned into an EVO by Van Kleiss, Holiday guided Rex by telling him everything he needed to do in order to cure Noah in time. Noah is also a reliable source for Holiday, since they sometimes ask him for Rex's location just in case he knows anything. Holiday even supported Noah in one of his pranks towards Rex and Bobo that officially made him win a pranking game.
- "She's there, isn't she?" — Holiday to Rex.
Holiday and Circe haven't really exchanged words with one another, however, she stated that she thinks Circe is very pretty. She also showed much interest in Circe and Rex's relationship with one another, occasionally asking Rex about her. Holiday also knew Circe well enough to know when Rex indirectly spoke about her.
Witnessing Dr. Branden Moses' cure, Rebecca was desperate to get a cure for Beverly. Calling him to Providence, she excitedly ran to him in order for him to examine Beverly. Eventually she found out that it was a fraud. He even admitted to fooling people that his cures actually worked. Rebecca, who was shocked at this, tried to get Beverly back but was too late.
Later, after Holiday threatened to kill Branden, he cowardly confessed that Beverly could still be cured. Although her sister was successfully cured, it is unknown whether or not Rebecca still feels great resentment towards him.
From the beginning, Holiday broke bonds with Providence ever since Black Knight moved in as Providence's leader. Black Knight and Holiday appeared to have some sort of rivalry against one another. They each constantly tried to prove themselves for some reason. Although their relationship started out rough, it worsened, turning them into enemies. Previously, the two use to do one another favors and save each others lives on the brink of demise. 
Holiday disliked Black Knight especially after she blew up the Hall of Records. Knowing, that Holiday wanted to investigate it and that it meant something very valuable to her, caused more animosity in their relationship. It's strongly noted that she rebelled against Black Knight since Holiday's motives from the beginning were to initially stop killing EVOs. She helped infiltrate new Providence and escaped with an obvious look of satisfaction.
- "I know you can't have it all, Breach. Rex doesn't belong to you! If you want this to stop, it's easy. Let him go!" — Holiday to Breach
Breach and Holiday started out as enemies. Holiday had Breach captured because she was the last foe to encounter Rex when he disappeared. She restrained Breach and interrogated her to the point where she confessed, but gave a difficult time in giving Rex up. Holiday was willing to get Rex back, even if it meant hurting Breach. In the end of the situation, she got Rex back, felt remorse after Breach was terribly hurt after all of her properties in her dollhouse were destroyed. Later, Holiday somewhat showed some sympathy to Breach when she was trying to be recruited into Providence by Rex and even attempted to hold White Knight off her back by Rex's request.
- "Listen carefully, Weaver! unless you want to be responsible for the loss of Providence's number one asset and unleashing the deadliest concentration of active nanites since the event; you will move out of my way..." — Holiday to Weaver.
When Holiday rushed Rex to Paradise in order to rid of his overloaded nanites. Holiday and her team bumped into Weaver, who wouldn't let the group past. As Weaver told them to turn back, Holiday refused and threatened him, saying that delaying Rex's offload could cost him his job and possibly something much worse. Eventually, he let her past.
Weaver managed to knock out Holiday and her team by giving them a knock out gas and disarmed and locked her in a closet with Agent Six and Bobo Haha. Eventually, he was revealed to be working alongside Skalamander and Biowulf. Much to her shock, Holiday discovered that Weaver had been mysteriously reactivating Rex's nanites to sell to the Pack, feeling betrayed. After Weaver went EVO Holiday waited on the sidelines in a scout ship, hoping that Rex and Six would destroy him. Unfortunately, he was too powerful. Holiday flew in and slammed the ship upon Weaver's head, knocking him unconscious.
- "And that was for the kiss!" — Holiday to John Scarecrow.
Ever since John Scarecrow played with Doctor Holiday's emotions, by posing as Agent Six to kiss her, she despised him. With all of the hostility he caused while infiltrating The Plant, Holiday grew a strong hatred for him, eventually forcing her to fight him and easily knock him unconscious.