Daniel Koster was an engineer in Vault 87.
He was stationed in a small engineering room off the atrium's upper level. He grew more and more suspicious of the secret experiments conducted in the vault, eventually choosing to confront the responsible people after the mysterious death of his wife.
His safe is unlocked by accessing the last entry of his terminal.
Interestingly enough, his wife, Mercia Koster, wasn't the subject of any experiments with FEV. Medical research terminal lists her as dead with code "N009", which means she had a I type cancer.
Daniel Koster is mentioned in Fallout 3, but does not actually appear in the game.