Kardio (カルディオ, Karudio) and Souza (サウザー, Sauzā) are one of the ally teams during the Faudo arc. Their ability isice armor and speed.
Another horse mamodo like Ponygon, Kardio was first introduced as a rival for the fire horse. At some point in the past, the team was ambushed by two allied Mamodo named Snail and Maraion, taking turns to attack Kardio, which they felt was not right. Although Kardio and Souza defeated the two mamodo, the battle left them with a negative prejudice regarding "teamwork" and they are highly hesitant to work with others.
Kardio and Souza's first meeting is shown to be when Kardio came upon Souza's campsite on Mongolia and Souza attempted to ride him. Kardio tried his best to unbuckle Souza but Souza held on tightly to his mane, tearing out a fistful of his mane on accident. While it is not shown, it can be speculated that Souza managed to tame Kardio after much effort and became his bookkeeper as well. Despite being animal and human, the two shared a close relationship and Souza is very fond of Kardio.
Like Ponygon, Kardio also only speak in animal sounds (Paru Paru Moon), yet Souza knows his real name. Kardio is one of the few mamodo who can understand Ponygon and knows his real name. During their first confrontation in Hokkaido, Souza mocked Ponygon's reliance on his allies after Kardio smelled their scent on Ponygon. Kafk Sunbeam, Ponygon's bookkeeper, arrives shortly after and they start battling. The fight is even until Souza uses "Dio Gikor Gidoruk", granting Kardio an ice armour and the ability to manipulate ice. With his new ability, Kardio easily breaks Ponygon's armour and proceeds to attempt to burn his book. However, Ponygon grabs onto Kardio's hoove, refusing to let them go on and Sunbeam uses this as a distraction to escape in hope of being able to buy some time for restoring his strength from within but Kardio freezes his legs, rendering him immobile. Kardio kicks Ponygon away and reaches towards Sunbeam for the book but Ponygon grabs onto his hoove again and his determination to return home safely to Zatch, no matter what, earns him his third spell, "Dioemur Shudoruk". Sunbeam starts reciting the spell but Kardio engulfs him with an icy wind in hope of halting him from chanting the third spell. Convinced that they had won, Souza and Kardio goes to burn Ponygon's book but suddenly realize that the snow around them was thawing. They then turn around and see Ponygon emitting flames from his body. The new spell grants Ponygon a new armour and the ability to manipulate flames but Ponygon's inexperience with his new spell causes Sunbeam to end up getting hurt by his flames and Kardio and Souza uses this to their advantage by attacking Ponygon whilst he steers his flames away from Sunbeam. A weak Sunbeam advises Ponygon to concentrate on the enemy and flare all his fighting spirit at them, further adding that they fight with heart, not technique. Kardio lands another blow on Ponygon, causing Ponygon to collapse. Ponygon then remembers Zatch and his friends and stands up again but Kardio angrily strikes him. Ponygon then remembers Sunbeam's advice and flares all his fighting spirit towards Kardio. Ponygon unleashes a fire clone at Kardio and Souza to distract them while he escapes with the unconscious Sunbeam but Kardio and Souza, though surprised at his ability to do so, figures out his trick and tries to attack him. Ponygon blocks their attack with a wall of flames without injuring Sunbeam, which shocked Souza, since he and Kardio had spent half a year lerning to use their power to that extent. Souza commands Kardio to pursue the duo but Kardio hesitates and admits to Souza that he had been afraid of Ponygon's eyes and was thus, tricked by Ponygon's fire clone completely. Souza realizes that they had lost this time but vows that next time, they would defeat Sunbeam and Ponygon.
In the Faudo Arc, Souza and Kardio appear again along with Arth and Ellie when Momon, Zatch and Ponygon carry their sleeping bookkeepers to the room in which the device that will send Faudo back to the mamodo world is kept. Arth states that he and Souza will eliminate all that stand in their way and proceeds to eliminate the group. Souza immediately uses "Diogikor Gidoruk" and sends an icy wave to the sleeping bookkeepers but is intercepted by Momon's "Ora Norojio". Kardio and Souza then start battling Sunbeam and Ponygon whilst Zatch and Kiyo try to reason with Arth. Kiyo uses "Rauzaruk" and Zatch manages to defend himself against Arth's sword. Zatch tries to persuade Arth to allow them to use the device but Arth triumphantly revealed that he had already activated the device that will send Faudo back to the mamodo world. Sunbeam angrily asked Arth if he was selfish enough to sacrifice the victims of Riou's curse but Arth pulls away his cloak to reveal a cursed Ellie, angrily stating that sending Faudo back to the mamodo world before his seal was broken was entirely Ellie's decision. At this point, Souza is notably upset, showing that despite agreeing to aid Ellie in her plan, Souza is still against her decision to end her life in order to save the world. It is assumed that Souza has a crush on Ellie because he was very worried about her when she was in pain due to Riou's curse. Kafk mocked Souza saying "I thought you didn't like working with allies." Calling a truce with Zatch's group, Arth and Kardio ran off to protect the device in Faudo's chest to send him back to the Mamodo world. They get into a fight with the Heart Guardian all the while defending the device. The battle proved difficult, partly due to Kardio's refusal to work in synchrony with Arth. After much convincing from Ellie, the battle took a better turn until the monster organ utilises his fire abilities. In the end, Kardio launches a suicide attack on the Heart Guardian by freezing from the inside out using his strongest spell Diō Gikorio Gidoruk. Kardio survived the attack however, as Souza allowed one of the Guardian's flames to strike their book prior to the attack, allowing Kardio to return to the mamodo world before he becomes completely frozen.
After Kardio had returned to the mamodo world and Faudo had been stopped, Souza returns to his home, which appears to be a campsite, where it is believed, that he is reunited with his sister.
Japanese Name: Diou Gigorio Gidoruku (ディオウ・ギゴリオ・ギドルク)
Description: A strengthening spell which bestows Kardio with a new ice-based armor upgrade.
Kardio's spells are variations of Ponygon's Spells. The "Shu" in Ponygon's spells is replaced with "Gio". Instead of the "Emur" in "Dioemur Shudoruk", Kardio has "Diogikor Gidoruk". However, the Doruk ending always remains the same. Both of their second spells begin with Go. Their third spells both begin with "Dio"