Vali Team
Vali Lucifer
Initially, it was said that Vali and Tobio hate one another, but it is was revealed in Volume 19 that Vali seems to share an older brother-younger brother type of relationship with Tobio. In the past, Tobio and Vali lived together, so they know each other quite well. According to Tobio, Vali is very clingy, comparing him to a troublesome little brother, so he avoids seeing him.
Fallen Angel
Azazel
Tobio is shown to respect Azazel, who has under his care since four years ago. He follows his orders faithfully and has worked behind the scenes. Tobio was greatly saddened when Azazel is sealed.
DxD (Team)
Lavinia Reni
Lavinia is a member of his Slash Dog team. Tobio has known Lavinia since prior of the series and was also the one who taught him about Sacred Gears and how to fully awaken the power of his Canis Lykaon. They have a very close friendship. Lavinia often call him by his nickname she given him "Tobi". And like Vali, he's also fearful of her.
Main Characters
Issei Hyoudou
Tobio meets Issei in Volume 15, alongside Dulio and Griselda. He offers Issei a sparring training and is friendly towards each other. Tobio is shown to be very grateful towards Issei for handling Vali and have a friendly talk about him.
Akeno Himejima
The two have not shown to interaction with each other, but they were aware that they related by blood as cousins. In the short story Hyakki Yako and Pandemonium, Tobio does seems to care about Akeno as he sends Jin to assist her against Kamaitachi and Kumowatari.