Appearance: All-Select Comics #1 (Fall, 1946).
Golden Age Appearances: All-Select Comics #1, Blonde Phantom #12-22, Marvel Mystery Comics #84-91, Sub-Mariner #25-28, Blackstone the Magician #2-4, All-Winners Comics v2 #1, Namora #2, Sun Girl #2-3.
Modern Appearances: She-Hulk v2 #2-3, 4, 7-8, 10-11, 13-25, 28-35, Avengers #325, Hulk Annual #16.
Dates Active: 1946-1949.
By day Louise Grant is the secretary for Mark Mason, the owner and sole operator of the the Mark Mason Detective Agency. By night Louise puts on a domino mask, a long red evening gown, lets her hair down, and fights crime. That was it for her background; during the Golden Age she was never given so much as an origin, despite her numerous appearances and general success as a crimefighter. She had no superpowers but was athletic and a good fighter. She was also on friendly terms with "Father Time, the Grim Reaper," a "quasi-angelic/ghostly figure" (to use Ronald Byrd's phrase) who was revealed to be responsible for selecting those destined to die. The Phantom's blasé attitude towards "Father Time," and her casual acceptance of his status, makes one wonder what other occult knowledge and experience she might have had during the Golden Age.
In the modern era she was brought back as a supporting character in She-Hulk, and it was established that in 1949 she married Mark Mason and had a daughter with him. (In her original appearances Louise had had a crush on Mason, but he'd only ever had eyes for the Blonde Phantom.) This daughter operated as the Phantom Blonde and worked as the She-Hulk's sidekick. She-Hulk v2 #22 portrayed Louise as working alongside the All-Winners Squad. During the events of She-Hulk Louise is rejuvenated and left the same age as she was during her original run. During She-Hulk she also gained the ability to break the Fourth Wall and mess with the readers' minds.
Note: Since the Phantom, as far as we know, never had an origin, it might be theorized, as Ronald Byrd points out, that Louise Grant was inspired to fight crime as the "Blonde Phantom" by the Blonde Phantom (I).