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Lieutenant Commander Dmitri Valtane was a science officer aboard the USS Excelsior under Captain Hikaru Sulu during the late-23rd century.
From 2290 to 2293, Valtane served during the Excelsior's three-year mission cataloging gaseous planetary bodies in the Beta Quadrant. (Star Trek VI: The Undiscovered Country)
Valtane was the bunk-mate of Ensign Tuvok in 2293. As an eager young officer, Valtane expressed both approval and surprise regarding Captain Sulu's recklessness in attempting to rescue Captain Kirk and Dr. McCoy from Rura Penthe.
Following Captain Sulu's abortive attempt to rescue Kirk and McCoy, the Excelsior came under attack while attempting to enter Klingon space. Gravely injured during the attack, Valtane, while in a state of clinical death, unwittingly transferred a memory virus to his attendant crewmate Tuvok. (VOY: "Flashback")
Background
Valtane was played by actor Jeremy Roberts. His character, who had no first name in Star Trek VI: The Undiscovered Country, appeared to die in "Flashback". However, one might speculate either that something other than death may have triggered the memory-virus transfer to Tuvok or that Valtane was resuscitated in sickbay shortly afterwards (or perhaps Tuvok's memory wasn't quite perfect). Any of these would explain Valtane's otherwise contradictory appearance, alive and well, on the bridge during the departure shot of the Excelsior's crew at the end of Star Trek VI.
The costume worn by Roberts as Valtane in "Flashback" was auctioned off in the It's A Wrap! sale and auction.
Apocrypha
Peter David's novel The Captain's Daughter states that Valtane was the Excelsior's first officer; the novelization of Star Trek VI states this as well. This has neither been confirmed nor contradicted in canon, although his sharing bunk space with Ensign Tuvok might seem to argue against it.