1) Someone who voluntarily decided to stay where they are (i.e. for life).
2) (Mil.) An enlisted soldier or officer that decides to re-enlist to go career
1) This place has good pay, great benefits, and nice people. They might just make a lifer out of me.
2) "Who's that?"
"That's Captain Anderson; he started out in the 22nd Armored, but ended up here as a tech. specialist. He's thinking about becoming a lifer."