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Regan is the younger (and potentially less wicked) of Lear's two evil daughters. Slightly more passive, Regan can't oppose her father on her own—she waits for Goneril to show up and support her. She's more likely to get men to do her dirty work for her than to attend to it herself (check out the torture scene, in which she eagerly begs Cornwall to pluck out Gloucester's other eye.)
As we have already been reminded that evil trumps good in this play, we are told in addition that really evil (Goneril) trumps mostly evil (Regan) when the former poisons the latter. This reinforces the self-centeredness-is-thicker-than-blood theme. Goneril and Regan weren't sticking together for sisterly values; they were doing it to beat Lear. Once Lear was bested, they turn on each other.