' s ▁do ppel g anger ▁from ▁the ▁parallel ▁universe ▁returned ▁in ▁the ▁season ▁finale ▁" Over ▁There ". ▁ ▁Despite ▁initial ▁reports ▁stating ▁guest ▁actress ▁Ther esa ▁Russell ▁would ▁be ▁playing ▁a ▁rec urr ing ▁character , ▁the ▁episode ▁marked ▁her ▁only ▁appearance ▁as ▁an ▁adult ▁version ▁of ▁the ▁character . ▁Russell ▁plays ▁Reb e cca ▁K ib ner , ▁a ▁former ▁test ▁subject ▁and ▁love ▁interest ▁of ▁Walter ' s ; ▁a ▁younger ▁version ▁played ▁by ▁actress ▁T eg an ▁M oss ▁is ▁seen ▁in ▁the ▁season ▁premi ere ▁" A ▁New ▁Day ▁in ▁the ▁Old ▁Town ". ▁In ▁the ▁DVD ▁audio ▁comment ary , ▁the ▁writers ▁explained ▁that ▁when ▁they ▁were ▁looking ▁for ▁an ▁actress ▁to ▁play ▁the ▁character , ▁the ▁casting ▁director ▁April ▁Web ster , ▁acting ▁on ▁Jeff ▁P ink ner ' s ▁instructions , ▁called ▁K ib ner ▁both ▁" a ▁bless ing ▁and ▁a ▁cur se ," ▁because ▁the ▁writers ▁wanted ▁to ▁explore ▁the ▁bur den ▁of ▁what ▁it ▁meant ▁to ▁be ▁special . ▁One ▁writer ▁explained ▁K ib ner ▁and ▁Walter ' s ▁past ▁relationship ▁had ▁not ▁been ▁a ▁long - term ▁affair , ▁but ▁instead ▁more ▁of ▁a ▁" l ittle ▁fl ing ". ▁The ▁episode ▁also ▁marked ▁the ▁first ▁appearance ▁of ▁Canadian ▁actor ▁Ryan ▁Mc Donald ▁as ▁rec urr ing ▁character ▁Br andon ▁F ay ette , ▁a ▁Mass ive ▁Dynamic ▁scient ist . ▁According ▁to ▁the ▁episode ' s ▁writers , ▁the ▁character ▁is ▁based ▁on ▁a ▁man ▁who ▁works ▁at ▁the