Like most of the members of the Vali Team, Arthur has great respect and trust towards Vali.
Arthur's sister. Not much is known about the depth of their relationship however, it is shown that the two get along quite well. In Volume 15, it is shown that he cares for his sister and wants her to return to everyday life and believes staying with Issei can make that happen. He personally requested for Issei to accept Le Fay in Volume 16 to be his contracted Magician, and Issei accepted his request to which he asked Ravel, and she said that since the Vali Team has been pardoned of their crimes, she is now a legitimate Magician to be hired.
Siegfried viewed Arthur as his rival, both being great swordsman and having weapons of opposite natures. Arthur, however, didn't view Siegfried as his rival, constantly searching for a suitable opponent for his Holy King Sword.
Arthur has a deep trust in Issei, requesting him to accept his sister, Le Fay as his contracted magician for the purpose of protecting her and be able to return home to her everyday life, believing Issei can make that happen due to his status as a hero in the Underworld. Arthur states that he will repay his gratitude to Issei.
Originally, Arthur was impressed with Yuuto and his Holy Demonic Swords. After Yuuto defeats Siegfried and gained all five of his Demon Swords, Arthur starts to see Yuuto as a worthy opponent for his Holy King Sword and tells Yuuto that he will battle him when Vali has his fight with Issei.
The woman Arthur is in love with and the reason he left the Pendragon household; to protect her from any punishment from his father, the current Head of the Pendragon family.