Ricardio (full title Ricardio the Heart Guy) makes his first appearance in "Ricardio the Heart Guy." Finn and Jake first come across him bestowing a Best Friend Massage upon Lumpy Space Princess. He is Ice King's heart come to life.
Finn shows a rivalry with him due to the fact that Princess Bubblegum seems somewhat attracted to him, and Finn, at the time, had a crush on her. This is shown when Finn says to Jake, "He's talking about science, man! I can't compete with that!" This obviously made him jealous. Finn punches Ricardio in the face once he suspects that he is evil, as he was seen throwing Ice King into a dumpster and carrying rope and broken glass bottles. Ice King then reveals to Finn and Jake that after an experiment had gone awry, his heart left his body, and Ice King began to grow weak. He somehow managed to crawl to the Candy Kingdom and tried to convince Ricardio to return to his body. But instead, Ricardio dumped Ice King into a dumpster, and left him for dead. Ricardio's plan was to cut out Princess Bubblegum's heart and force it to marry him. His idea of the forced marriage was similar to that of the Ice King's usual plans. Ricardio ties up Princess Bubblegum later on in the episode and Finn and Jake save Princess Bubblegum. He is returned to Ice King's body at the end of the episode, and says that he will be back.
He then returns in season four episode, "Lady & Peebles." Having decided he wants to marry Princess Bubblegum instead of just her heart, he escaped Ice King again and made himself arms and legs from Ice King's body. He hid in the black ice cave in the Ice Kingdom and kidnapped Finn and Jake in order to lure Princess Bubblegum to him. When she finally came, Bubblegum agreed to marry him if he could defeat her in hand-to-hand combat. Since Ricardio was a heart (the weak point of a body) Princess Bubblegum was easily able to defeat him and he crawled off in defeat. Ricardio revealed that he knew who Ice King was before he wore the crown by mentioning his old name (Simon) when he walked on him as a sign of disrespect, since he was a body part in Ice King and possibly "sharing" his memories.
Ricardio was the heart of the Ice King. However, instead of looking like an actual heart, Ricardio's body has the shape of the iconic heart symbol that represents "love." Ricardio is fairly small, smaller than that of the small Candy People. He also has a very detailed face (unlike most of the characters seen in the show). He has bold eyebrows, eyes that are depicted as actual human eyes instead of dots, bold nose, wide lips, and cheekbones. Ricardio also possesses arms and legs.
When Ricardio created a new body in "Lady & Peebles," he used sinews to connect himself to bulging arms and legs, though when Princess Bubblegum fought him she ripped off one leg and one arm. He also has a smiley face mask taped on top of his head. In this episode, his eyes appear larger, only his upper lip is shaded, and his face is slightly less detailed and defined.
In "Gumbaldia," Ricardio appears as he did in his debut and joined Gumbald's army to attack and conquer the Candy Kingdom.
Ricardio is the heart of the Ice King, but much more evil (the Ice King usually causes trouble unintentionally). Because of this, Ricardio possesses some major traits from him. Mainly, Ricardio has an obsession for princesses. Unlike the Ice King however, Ricardio literally wants the hearts of princesses, especially Princess Bubblegum's (as seen in "Ricardio the Heart Guy"). However, this changes in "Lady & Peebles," in which he claims that he didn't just want Princess Bubblegum's heart, he wanted "the full package." Some traits that Ricardio possesses that Ice King does not is that he is social and charismatic. The Candy People and the princesses take a liking to Ricardio while they despise the Ice King. Ricardio can also be violent and malicious. In this debut episode, he beat up the Ice King (his host) and threw him into a dumpster, and in "Lady & Peebles" he used Ice King's own organs to make a body for himself, and he later showed it off by twisting an unconscious Lady Rainicorn into a knot and throwing her against a wall. This also proves that Ricardio has no desire to keep Ice King alive by being his heart; he wants a life of his own.
- Unlike Ice King, Ricardio is charismatic enough to become popular and well-liked among the Candy People and princesses.
- His name is a pun: Ricardo is "Richard" in Spanish and Portuguese, and cardio comes from cardiac, which means "of or related to the heart."
- Even though Ricardio is an actual heart, he doesn't look like one. Instead, he just looks like the "love" heart design. However, his early designs show him looking like an actual heart.
- He is one of the few characters which have many details in the face, while the others, even the main characters, have simpler faces.
- Because he's the heart of Ice King, he should be at least as old as him. This was pointed out in the Adventure Time: My Two Favorite People DVD special feature "Little Did You Know." His "species" is shown to be a human heart, meaning the Ice King could still genetically be a human.
- Ricardio seems to be too big to be the Ice King's heart, but later in "Ricardio the Heart Guy," he seems to shrink to the correct size.
- Ricardio appeared later on in the series during a commercial on Flicks where Finn and Jake (along with various other characters from The Amazing World of Gumball, Adventure Time, and Regular Show) are in a movie theater, and Jake is annoyed by a beating noise. Later, it is revealed that the beating came from Ricardio, who was sitting next to Jake, watching the movie with a surprised look on his face.
- On Ice King's papercraft robe, Ricardio's face peeks out a little bit.
- In the storyboard for "Lady & Peebles," the smiling piece on his head was the head of Homer Simpson from "The Simpsons." This was removed as it was a storyboard joke.
- He calls the Ice King "Simon."
- Ricardio appears in "All the Little People" as one of the Little People.
- In the episode "Gotcha!" a picture of Ricardio appears on Lumpy Space Princess' diary.
- He resembles Majora's Wrath, the final boss from the game The Legend of Zelda: Majora's Mask.
- He along with The Earl of Lemongrab, Lumpy Space Princess, Tiffany, Magic Man, Donny, and Squirrel were pictured in the credits of "Water Park Prank" as one of those who can't return to Wet Pipes Water Park.