Meredith Ragen is the main villainess from NCIS episode 13.14, "Decompressed."
She was played by Maya Stojan, who also played Agent 33 on Marvel's Agents of S.H.I.E.L.D.
Meredith is a former Army parachutist who was discharged after she murdered a translator with her bare hands (mistaking that person for an enemy). Her discharge from the Army prevented Meredith from facing murder charges. In the episode, Meredith was part of a crew of divers involved in a PPA test, which was revealed to include using vials of liquid red cocaine. However, Meredith was revealed to be part of a drug-selling scheme, when Diego De La Rosa caught her stealing the vials. To keep him quiet, the evil Meredith killed De La Rosa by tampering with his air hose, which caused him to flip out due to lack of oxygen and die within minutes.
Meredith was caught and revealed as the killer due to a rash on her upper lip, which was part of Meredith's allergy to anything with red dye. Meredith was eventually arrested for her crimes.