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L.L. are the initials for many Superman characters who, for the most part, have tended to be his adversaries, love interests, or relatives.

Especially in Silver Age comics, it is a continuing trend to name characters with these initials.

Some such characters are:

Linda Lee, the original secret identity for Supergirl, Superman's cousin.
Lana Lang, Superman's first girlfriend.
Linda Lang, Supergirl's secret identity.
Lois Lane, Superman's main love interest and (before the New 52 reboot) wife.
Lucy Lane, Lois' sister.
Lex Luthor, Superman's archnemesis and frequent foe.
Lena Luthor, Lex's sister Pre-Crisis; Lex's daughter Post-Crisis.
- Tess Mercer, birth name Lutessa Lena Luthor, Lex's half-sister in Smallville.
Lori Luthor, Lex's niece.
Lionel Luthor, Lex's father on Smallville.
Lenny Luthor, Lex's nephew in Superman IV.
Lucas Luthor, Lex's half-brother on Smallville.
Lori Lemaris, Superman's college sweetheart.
Lenora Lemaris, Lori's sister.
Lyla Lerrol, Superman's love affair on Krypton.
Letita Lerner, Superman's babysitter.
Lara Lor-Van, Superman's biological mother.
Luma Lynai, one of several characters to have used the name Superwoman
Linda Lake, a reporter appearing on Smallville
Lesla Lar, a Silver Age foe of Supergirl
Liri Lee, member of the Linear Men (uses the pseudonym Liesel Largo)
Lyrica Lloyd, from Superman #196 (May 1967)
Lola-La, a member of a Neandertholic people on a remote undiscovered island (with dinosaurs) whom an amnesiac Superman nearly married.
Lupé Leocadio, a Metropolis police officer
Leslie Luckabee, posed as Lex Luthor's son in Lois & Clark: The New Adventures of Superman

Leonore Lemmon (May 11, 1923 - December 30, 1989) was an American socialite who was the fiancée of actor George Reeves

Possible Explanations

In The Adventures of Superman #646, Mr. Mxyztplk suggested that the initials had an important significance within the Kryptonian Language.

In Smallville, Chloe Sullivan, who has a long-time crush on Clark, sometimes jokes about how Clark is only attracted to people with the initials LL, referring to Lana Lang and Lois Lane, and possibly Lex Luthor when they were still friends.