Mat Kowalski was a male crew member of the UNSC Apocalypso, who flirts with Arrelts. He appears to be knowledgeable of the AI systems such as Melissa. He first appears 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 probably, though not definitely, Kowalski. His death therefore occurred sometime before October 20, 2552. It is likely that he was reassigned to another vessel after the Apocalypso and was killed in battle. Another possibility is that he died in an "accident" like Captain Greene, however this seems unlikely due to Greene's absence from the Memorial.