Karl Yamaji is a supporting character in the Cardfight!! Vanguard G series, specifically known for his role during the Stride Gate-hen arc. He is a member of Team Trinity Dragon, fighting alongside his teammates Tsuneto Tado and Kei Nagara. The team's name originated from a derogatory remark by Kyou Yahagi, who called them third-rate dregs; Karl reimagined the insult by replacing the word for three with Trinity, which solidified their collective resolve.

Karl is a strategic and justice-driven fighter who uses a Dimension Police deck, with a particular focus on the Enigman clan of units. His primary strategy in cardfights revolves around powering up his vanguard, often utilizing the skills of the unit Great Galactic Governor, Commander Laurel D in the later stages of a game. He supports this approach with a versatile lineup of trigger units, including critical, draw, stand, and heal triggers, and further refines his tactical coordination through the use of G units. His battle cries, such as Judge with the hammer of justice. Protect the dimension's future and Listen to the voice of justice. Surpass time and space and unleash, underscore his commitment to heroic principles and the theme of dimensional guardianship that runs through his fighting style.

Throughout his known history, Karl has achieved significant victories, including a solo win against Taiyou Asukawa and team victories against the trio of Chrono Shindou, Tokoha Anjou, and Shion Kiba. He has also faced defeats, notably against Sousuke Wakamizu and Ryutarou Oyama. Beyond these competitions, Karl proves to be an instrumental fighter against interdimensional threats known as Relics, emerging victorious in those critical clashes. His surname, Yamaji, holds the meaning of Mountain Path, which is an anchor point for his identity within the series.