Charlene Masters

From Memory Alpha, the free Star Trek reference.

Lieutenant Charlene Masters was a Starfleet Sciences division officer with engineering responsibilities aboard the Enterprise in 2267.

Masters was in charge of re-amplifying the starship's dilithium crystals after a near-collapse of the known universe drained them of power and put the ship at risk. Identical beings from opposite universes each desired the crystals for their own purposes. Anti-Lazarus rendered her unconscious and stole a pair of crystals as part of his plan to prevent his mad opponent, Lazarus, from destroying both of their universes. Later, Lazarus created a fire to force her out of engineering to steal his own dilithium.

Along with her assistant, Masters shared a low opinion of Enterprise coffee. (TOS: "The Alternative Factor")

Charlene Masters was portrayed by Janet MacLachlan. Although she was referred to as "lieutenant" in dialog, she wore no rank stripe (denoting an ensign). Perhaps lieutenants wearing no insignia were meant to be lieutenant junior grade.