[Locke.]

I agree that he was definitely not expanded on enough, but we found out everything there was to know about him in the end. He worked for Widmore getting people "where they needed to go". Abbadon wasn't from the wasn't from the past, but Widmore had met some of the people from Oceanic 815 when they travelled back to the 70's on the Island, plus Jacob had visited Widmore off the island and told him what he needed to do (which ended with him having to go back the Island in S6), which is what Abaddon would come in handy for (getting them to go back).

I agree he shouldn't have been killed off so suddenly, but he's not an unsolved mystery