Sobi is one of the two main antagonists of the Star Trek: The Next Generation episode "Symbiosis", along with Langor.
He was portrayed by Judson Scott, who also played Joachim in Star Trek II: The Wrath of Khan.
Sobi was a member of the Brekkian race. He and his colleague Langor were sent to supervise the delivery of a consignment of felicium to the Ornarans in return for goods. Felicium was used to combat a deadly plague on the planet: The entire population had it and felicium kept it at bay but needed to be taken constantly. However, the Ornaran ships were badly maintained and the ship burnt up in orbit when its engines failed. Sobi, Langor and two Ornarans, T'Jonn and Romas, were beamed aboard the Enterprise along with the felicium moments before its destruction.
A dispute soon broke out between the two groups as to the ownership of the felicium: The Ornarans argued that they had paid for it, but Sobi and Langor retorted that the payment had been lost in the ship's destruction. The Brekkians' economy was based around producing felicium for the Ornarans, just as the Ornarans' was based around producing all the goods the Brekkians needed. The argument at one point spilled over into violence, with Sobi and T'Jonn using their natural electric charges against each other and Lieutenant Yar having to separate them with a warning shot.
Sobi and Langor later summoned Picard and Crusher to their quarters and announced they had decided to give the felicium to the Ornarans after all. In the process, they gave away the fact that the Brekkians were aware of something the Enterprise had already learned: The felicium had already cured the plague, but the Ornarians had become addicted to it. If they went without it, they experienced withdrawals that they mistook for the plague, and the Brekkians were worried that they might make it through the withdrawals and realise the truth if they went without felicium for too long.
Since the Prime Directive forbade him to interfere, Picard was forced to keep quiet as Sobi and Langor completed the transaction... but also refused to provide parts to repair the Ornarans' ships, meaning they would soon be unable to transport the felicium shipments and the cycle would end. Sobi and Langor left the ship frustrated.