Yuuto is very grateful towards his master, Rias, for saving him after he died from the project called "The Holy Sword Project". Being faithful to his status as a Knight, Yuuto is very loyal to Rias and calls himself "Rias Gremory's Sword". He sees her as an elder sister, but doesn't feel comfortable calling her onee-chan despite Rias wanting him to.
Issei at first disliked Yuuto, because of his good looks and popularity with the girls, but as time passed they became good friends. Yuuto claims himself to be Issei's best friend as Issei was the one who helped him let go of his rage against the Holy Swords.
When Issei died, Kiba was greatly saddened and wanted to avenge him, but held back his emotions, preventing them from getting the best of him because he remembered the promise that he made with Issei which was that if one of them were to die, the other would protect the girls in the Gremory group with all their strength. Yuuto then fought against Siegfried and was enraged when Siegfried said, "Hyoudou Issei died in vain". Yuuto, along with the Gremory group, were finally able to kill Siegfried.
Issei means a lot to Kiba, after having been implied that Issei was Kiba's first best friend, comrade, and rival. When Issei's death came around, the event left even Kiba on an emotional turning point.
Yuuto and Xenovia were originally on bad terms due to Yuuto's hatred on the Excaliburs and Xenovia's position as an exorcist. After Yuuto obtained the Sword of Betrayer and Xenovia became Rias' Knight, the two of them became closer and have started to train together.
Yuuto and Akeno have a neutral relationship with each other. They are both loyal servants to their master, Rias Gremory. Since Akeno is the Queen in their group, Yuuto treats her with a high honor of respect and honesty.
Yuuto and Koneko are shown to have a close relationship. This was shown in Volume 3 when she was willing to go along with Issei's plan to team up with Irina and Xenovia to destroy the remaining Excalibur fragments without Rias' permission in order to keep Yuuto from becoming a Stray Devil.
Kiba and Gasper live together in a mansion close to Issei's house. When Gasper was still a shut-in, Kiba (along with the rest of the group) helped him gain more confidence in himself and his power.
He is Yuuto's swordmaster. Kiba has shown to have much respect towards Okita and still call's him "Master" despite Okita no longer teaching him in swordsmanship.
After losing to Yuuto in their master's Rating Game in Volume 5, Tsubaki had developed feelings for him to the point of her losing her usual calm demeanor when in front of or at the mention of him. Yuuto is currently unaware of her crush towards him.
Arthur became, as of Volume 12, Yuuto's rival after being impressed by him, his Holy Demonic Sword and obtaining Gram as well as the rest of Siegfried's Demon Swords after defeating him, seeing him as a worthy opponent for his Holy King Sword. Arthur tells Yuuto that he will battle him when Vali has his fight with Issei.
Kiba hates Freed with a passion shown in the Light Novel and in the anime, such as in Volume 4 when he finds out that Freed was experimented on to wield a Holy Sword, as such, he feels a certain amount of hatred towards him because his fellow fallen comrades died because of this experiment. Yuuto is strongly disgusted by Freed's vulgar words and showed no signs of remorse after finishing him off in Volume 6.
Tosca was a close friend of Kiba during ordeal of the Holy Sword Project. She had made her return years later and Kiba was overjoyed when he found out that she had survived resulting in him running to Tosca and hugging her, as he was very happy to see his old friend.