From Halopedia, the Halo wiki
Kowalski  was a male crew member of the UNSC Apocalypso, who flirted with Arrelts. He appears to be knowledgeable of the AI systems such as Melissa. He first appeared in the memory that Melissa uses for her benchmark test in the Phase 2 Operator's Monologue.
The name "Mat Kowalski" appears on the ONI Memorial at ONI Alpha Site in New Mombasa. This is possibly, though not definitely, referring to Kowalski. His death may have therefore occurred sometime before October 20, 2552. However, it should be noted that the memorial may not be entirely factual, as it contains names such as Margaret Parangosky, who is known to have survived despite being listed as dead, along with every other member of ONI named at the time of the release of Halo 3: ODST.