Claire - played by Emilie De Ravin
AKA - The Angry Mother
On Island - Claire was about to give up her baby for adoption when Oceanic 815 crashed. She gave birth on the Island to Aaron and formed a relationship with rocker Charlie (before he drowned.)
She has always had ties to Jack - although neither of them have known (until recently) that they're siblings.
Claire disappeared on the Island - and despite all attempts to find her, failed to show up.
In Lost: The Final Season, Claire appears to have been working as an emissary for Locke/ Man In Black - with the sole motivation of finding her missing son whom she believed had been taken by the Others. Angry and bitter, Claire has nearly killed Kate for taking Aaron off the Island - and appears to be heavily allied with "Locke."
Off Island - In this season's parallel storyline, Claire has been tied to Kate as their paths have intertwined. In this story, Claire is still pregnant, but the people who were adopting the baby have abandoned her.
Love Interest - Aaron and Charlie were Claire's love interests - and with one missing and one dead, she's lost a lot of faith.
Lingering question - Has Claire been infected beyond redemption? Can she break the thrall of working under the Man In Black and survive?