what piece of media is a character named Saeko from

Saeko is a name used by several fictional characters, but the best-known one is Saeko Busujima from Highschool of the Dead. Another well-known Saeko is Saeko Nogami from City Hunter.

Most likely answer

• Saeko Busujima → Highschool of the Dead

• Saeko Nogami → City Hunter

• Saeko Takami → the Persona series

If you meant a specific Saeko, that title depends on which character you saw.