Ian is Oliver’s birth father and the primary parent who had a stake in raising him. After Marissa had him, she took the necessary time to recover and returned to her active duties. Ian applied and was approved to be moved to Russell’s World to work at a Legion Medical Station there and was able to be closer to Oliver and take him on his 3 off cycles. 

Appearance: I’m thinking young Michael Caine with darker hair?  Probably not his voice though…