A Starfleet lieutenant posted to Relay Station 47 near the Klingon border, who by 2369 has been killed and replaced by a coalescent organism that copies the creatures it absorbs.
Lieutenant Keith Rocha figures in the sixth-season episode Aquiel, though he is never seen alive. The Enterprise-D finds the relay station deserted, with a patch of cellular residue on the deck, and at first assumes that his junior officer, Aquiel Uhnari, has been murdered.
Uhnari's logs paint Rocha as hostile and overbearing, while his own record is that of a decorated officer. When Uhnari turns up alive, the residue is identified as Rocha's and she becomes the suspect.
The truth is that the real Rocha was consumed at an earlier posting by an organism that took his shape. On the station it tried to absorb Uhnari, failed, and took her dog, Maura, instead, which is how Geordi La Forge finally meets it.