Mikaela Leishman was a Human female. She was an Ensign in Starfleet and served aboard the USS Defiant as an engineer, in both Main Engineering and on the bridge Leishman has a personality trait to discover what annoys high-strung people and then exploit it for her own amusement. She also hordes chocolates in a secret stash
In January 2377, Leishman served as the Chief Engineer of the Defiant during its pursuit of Taran'atar. After the Defiant was damaged by the detonation of ultritium in the comet Nahanas, Leishman was able to repair the warp nacelles. She later replicated the neuro-pulse device, that was designed by Nog and Julian Bashir to break Iliana Ghemor's hold over Taran'atar.
By 2381 Leishman had been promoted to Lieutenant and had transferred to the USS Aventine as Chief Engineer under the command of Captain Ezri Dax. She was aboard by the time the Aventine investigated the wreck of the NX Columbia. Ensign Nevin Riordan worked with her to interface the Aventine and Columbia computer cores; Leishman later recommended to Dax a commendation for the Ensign's work.
After the Aventine answered the distress call from the USS Enterprise-E in the Azure Nebula, Leishman worked with the Enterprise to interface their two computer systems, suggesting several possibilities to allow the Sovereign-class vessel the ability to meet the required computer throughput, which raised the ire of Miranda Kadohata when relayed through Geordi La Forge. Leishman later invited La Forge over to the Aventine to assist in modifying the ship for the ability to open and traverse the subspace tunnels discovered in the nebula; while the two reconfigured the vessel, she flirted openly with La Forge, and the two agreed to meet for dinner when the conflict was over.
After the USS Titan appeared at the Azure Nebula, the Aventine crew was introduced to former Earth Starfleet Captain Erika Hernandez; the Aventine crew also planned to assault, board, and capture a Borg scout craft in order to secure its vinculum. Second Officer Gruhn Helkara escorted Hernandez around the Aventine, including down to Main Engineering to meet Leishman, where she was doling out orders to her subordinates to prepare for the attack. Hernandez offered to assist in any way possible, and Leishman described that they were having difficulty maintaining their transphasic shields; Hernandez adapted their shield harmonic subroutines in a matter of seconds, leaving Leishman and Helkara stunned. Leishman's second issue was that they were not proficient in piloting their quantum slipstream drive; Erika's answer was to ask to pilot the ship herself.
The attack went off well enough, and Hernandez, Helkara, Kedair, and Leishman beamed over to the scout's vinculum in order to attach the Earth Starfleet Captain. In order to speed the process of connecting Hernandez, Leishman was ordered by Captain Dax to disconnect the feedback buffer, allowing her to infiltrate the Borg Collective and masquerade as the Borg Queen. Although their actions were momentarily successful, the Queen quickly reestablished dominance and transferred her presence to the scout, attacking the Aventine and its away team. When the vessel itself began to fight them, Hernandez controlled it back, and got the team out.