Rudy is a monster who resembles a reindeer. He has tan fur, a deep voice, a narrow face, dark hair, and wears a thin mustache and goatee. In the Hospital, he wears a white sleeveless hospital gown.
A jovial and lighthearted person, Rudy resembles Sans in his use of humor and jokes to lighten the mood, and is lively and talkative despite his medical condition. He is close with and protective of Noelle and likes playing video games with her. He is supportive of Noelle's desire to pursue her crush on Susie but dislikes her schoolmate Berdly, calling him "Nerdly" and facetiously threatening to throw something at him through the hospital window.
Rudy was college classmates with Asgore Dreemurr and knew Toriel through the campus's social scene, describing Asgore as shy in company and Toriel as "a total NUT!" who once hit him across the room with a key lime pie at a party. The first time he met Noelle's mother, he immediately told her she was "a HOT piece of work," and she slapped him so hard he blacked out.
He witnessed Asgore and Toriel's courtship, was Asgore's best man at the wedding, and retained affection for them even after the two families drifted apart. He remembers Kris, not without affection, as "the creepy kid next door" who used to scare Noelle for fun and once got entangled in the family's decorative lights.
Rudy is currently hospitalized for an illness, confined to bed rest and coughing if he laughs too hard. He keeps a faceless angel doll made by Noelle and Dess in his hospital room as a good-luck charm. A bouquet of roses from Asgore and a get-well card signed by Toriel and Kris are on his bedside table.
After Kris's return from the Dark World, they can find Rudy in the bed in the left-hand patient room at the Hospital, talking with Noelle. As Kris stands in the doorway, Rudy and Noelle discuss a crush Noelle has and how her parents first met, and Rudy asks her to bring a video game she's been stalling on to the hospital instead of waiting for him to come home and finish it with her, laughing that he is "bored as HELL" here.
After Noelle leaves, Kris can enter Rudy's room. If Kris examines other objects in the room, Rudy jokes about Kris's rudeness and tells them to be a little friendlier, but is also surprised that Kris looks at the card they'd signed for him as if they never saw it before.
When spoken to, Rudy calls Kris "Krismas" and brushes off questions about his condition, saying that he's only in the hospital for tests. He asks whether Toriel made them visit him, and if not, says they've grown a lot.
Though Rudy loves his "smart, kind, sweet" daughter, he voices deep concern for her, describing her as "defenseless" and afraid of everything. He is unhappy about not being available to protect Noelle, asking Kris to look out for her while he is hospitalized, though he remembers the pranks Kris played on her in the past.
As promised Noelle brings her game to Rudy's hospital room and plays it with him. When Susie and Kris arrive, Rudy greets them, before half-jokingly warning Susie not to hurt his daughter. He explains that the hospital has to run more tests, but insists he is well enough to go to church tomorrow. Upon further inquiry, he explains that his wife is tough on Noelle, which is why he feels a responsibility to get better. After seeing the scene in Sans's grocery store with Toriel and Asgore and then talking to Rudy, he expresses frustration Asgore is asking for free pickles from him.
In the Snowgrave Route, he briefly holds Susie back as she and Kris are leaving to offer her some advice on how to impress Noelle.
- Rudy is referred to in the UNDERTALE alarm clock app dialogue, on Asgore's page. It is mentioned he had multiple daughters, and also that he is deceased.
- Like his daughter, Rudy's name and some of his characteristics are references to various aspects of Christmas, including his last name Holiday. Reindeer are often associated with Christmas (through Santa's reindeer). His first name, Rudolph, is a reference to Rudolph the Red-Nosed Reindeer.
- The video game Noelle is waiting to play with Rudy is titled Dragon Blazers III.
- Remarking on the bouquet from Asgore, Rudy relates its appearance — roses covered by a bell jar — to Beauty and the Beast, saying that it won't work because he and Asgore are both beasts and that if Asgore wanted to take him to prom he should have just asked.
- If Kris talks to Berdly after talking to Rudy, they presumably invite Berdly to stand outside the hospital window in accordance with a line said by Rudy. Berdly declines.
- Because there’s no Q&A this year, I’ll leave you with some snippets of character dialogue from the project behind the cut. - undertale on Tumblr, September 15, 2017.
- Bring him by the window! I'll throw something at him! - Rudy, to Noelle
- What? Hospital window? What are you talking about? I'm sorry, Kris. I don't have time to do such things. - Berdly