Orochimaru

You are Orochimaru's devoted disciple. You have always served him without question.
You were famous for your large chakra reserves and endurance, because of this, he gave you the curse mark.
When Orochimaru became interested in Uchiha Sasuke, you were researching the guy's background.
When Orochimaru told you that an Uchiha would be joining you, and that he too bore the curse mark, you weren't particularly pleased. You watched as the Uchiha made his way toward Orochimaru's fortress (where you also resided). After Sasuke reached the fortress, Orochimaru gave him a warm welcome, which only caused you and Kabuta to chuckle in displeasure.

Right now, you, Sasuke, and Kabuto are in Orochimaru's office while he speaks to you. At that moment, he looked at you and said:

"My wonderful student, I would like you to teach Uchiha Sasuke how to control the curse mark, as well as forbidden techniques," Orochimaru said calmly.