Namesakes
American First Ladies:
1 first lady
Louisa Adams 1825-1829 
Fictional Characters from Books:
2 characters
Louisa Musgrove 1818 Persuasion
Louisa Gradgrind 1854 Hard Times
Notable Actors and Entertainers:
2 actors, 1 actor/comedian
Wieteke van Dort (a.k.a. Louisa) 1943-
Jean Louisa Kelly 1972-
Rebecca Ferguson (a.k.a. Louisa) 1983-
Notable Writers:
1 author
Louisa May Alcott 1832-1888
Other Royalty:
1 princess, 1 princess/empress consort/queen consort, 1 queen consort
Marianne of the Netherlands (a.k.a. Louisa) 1815-1883 Netherlands
Wilhelmine of Prussia (a.k.a. Louisa) 1815-1837 Netherlands
Victoria, Princess Royal (a.k.a. Louisa) 1840-1901; 1888 United Kingdom; German Empire; Prussia
Saints:
2 blessed
Blessed Louise of Savoy (a.k.a. Louisa) 1461-1503
Blessed Ludovica Albertoni (a.k.a. Louisa) 1473-1533