Not going to give away any spoilers unless you want to read this
Resident Evil wiki (read the section on "After Umbrella")
(*Spoiler Alert*)
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(*I did warn you*) 
Jill is a boss in Resident Evil 5, where Chris ends up fighting her, not to kill her but rather to save her, on which he does.
The reason why I know this, is because my younger brother owns a 360 and I watched him play that section of the game.