I found Lowell absolutely mesmerizing, in both looks and acting, wardrobe be damned. The sexiest female character on the show. It actually annoyed me that the writers felt it necessary to treat Jamie like Rey by constantly having characters gush about how she's so gorgeous. We know, we can see it, just move past it.
Other than her first or second episode where they had her outright yelling at Jack (if memory serves) there was generally a quiet, soft-spoken toughness to her, a no-nonsense, understated quality. Competence.
Someone (I think Jack) asks her if she's ever been to Niagra Falls, and she just blithely throws off this line about her brothers trying to throw her in, and then immediately gets back to business. There's none of Claire's naivete or uncertainty, and you can tell Jack's usual tactics with his ADAs are not going to work. And her EYES. And the hair. And the voice. I mean compare her soft, soothing tones to Abbie's hoarse, ex-sorority girl bar voice.
In her final episode, when she and Jack are sitting in his office and she tells him she's leaving, and he says "go out like you came in, you don't owe me anything," she seems honestly emotional but doesn't overdo it, much as it would be in real life, and I actually end up feeling something in a moment that otherwise could have been totally forgettable.
For the record, Abby struck me as the sanctimonious poser, and I still can't stand her. Not attractive in looks or personality, and trying too hard to be a hard ass.