Unfortunately, he's...not available at the moment.
~ Vantika, refusing Sisko's request to speak with Bashir.
Rao Vantika is a villainous Kobliad from the Star Trek: Deep Space Nine episode "The Passenger".
He was portrayed by James Harper in his original Kobliad body. When he possessed Julian Bashir's body, he was portrayed by Alexander Siddig.
Vantika was a genius who went in to medicine, and eventually became a medical supervisor of a high-security penitentiary. At some point he became obsessed with extending his own life at all costs, and began experimenting on prisoners at the penitentiary. After being exposed and forced from his position he became a criminal mastermind.
For the next twenty years Vantika evaded Kobliad authorities while raiding government labs to steal research on bio-regeneration. During that time he faked his death several times, fooling even highly trained medical experts. One such expert died after mistakenly assuming Vandika was dead.
As the Kobliad required deuridium to help stabalize their cell structure, some Kobliad such as Vantika turned to piracy to obtain the substance. During a trip to steal a shipment that the Federation had procured from the Gamma Quadrant, Vantika was captured by the Kobliad security agent Ty Kajada. While being transported back to the Kobliad homeworld Vantika started a fire on the ship which killed the pilot and seriously injured himself and Kajada.
A distress call was sent out, which Dr. Bashir and Major Kira Nerys answered. By then Vantika's body was dying. When Dr. Bashir got close enough Vantika grabbed him, transferring glial cells containing his consciousness into Bashir's body. His last words in his original body were, "Make me live."
DS9 personnel were skeptical to Kajada's warnings that Vantika had survived, even after the station computer was sabotaged in a manner similar to what it was on her ship. It was later discovered that Vantika had survived by transferring his glial cells into another individual. At first suspecting that Kajada was unwillingly housing Vantika's consciousness, the crew soon learned that it was Bashir who Vantika had possessed.
Using Bashir's codes, a band of smugglers led by Vantika took a runabout to meet the freighter Norkova that had the deuridum as it entered the Alpha Quadrant. Beaming aboard the smugglers killed the bridge crew, and then Vantika himself beamed aboard. Before Vantika could escape with the Norkova, Commander Benjamin Sisko ordered the ship held in a tractor beam. Jadzia Dax was able to send a pulse along the tractor beam that disrupted Vantika's control long enough for Sisko to get Bashir to lower the shields on the Norkova so they could transport him back to the station. Once beamed back to the station Sisko stunned Bashir before Vantika could reassert control.
In the infirmary, Dax used a special transporter to beam the glial cells out of Bashir's body into a containment field dish. After Bashir and Kajada recovered, Sisko and Dax debriefed them on what had happened. Bashir did not remember what he did while under Vantika's control, but felt humiliated over the experience. Sisko reassured Bashir that no one blamed him for what had happened.
Kajada then asked if custody of the prisoner Vantika had been formally returned to her. When Sisko answered in the affirmative, Kajada fired her phaser at the containment field dish, vaporizing it and destroying the last traces of Vantika before she walked out of the room.