Lt. Haggerty is disgruntled by the fact that she is guarding HELIOS One rather than being out on the front lines, though she does comment that if the NCR lose the battle at the dam, then the plant may be their last stand.
If you are wearing NCR armor when you first meet her and you fail the speech check, she will remark how your uniform has a lot of blood on it and you don't seem injured enough to bleed that much. She will then assume you killed an NCR member in order to get the armor, making her and all her guards will attack you.
She will die if you activate the Archimedes I Orbital laser.
She will allow you into HELIOS One if you have a positive reputation with the NCR.
In case Lt. Haggerty dies, she won't be able to give you the That Lucky Old Sun quest and can't give you the GPS coordinates during the Restoring Hope quest. The first quest can then be started by talking to Fantastic. For the second quest, it's not essential to talk to Haggerty, you can just go directly to the missing shipment, which is located northeast of the Southern Nevada Wind Farm and continue the quest from there on.
"Damned if I know. Put up a hell of a fight, though. We had them outnumbered, I don't know, twenty to one, maybe. They held out as long as they could. Lost most of their force before they retreated. Gave us a little parting gift, too. They had some of the plant running, but they shut it all down. Enabled an old security system, too, to keep us away from the controls. Jerkoffs." – Response to Courier asking why the Brotherhood was here
"Your being here is a start, soldier. We aren't exactly overflowing with troopers here. But if you really want to help us, talk to the idiot with sunglasses in the back of the plant building. He's been trying to get this place running for months and hasn't made any forward progress."