En fait, lui aussi l'aimait.

English Translation

In fact, he loved her too.

Where is the subject?

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The subject is lui, which here serves as the emphatic form for il. It goes naturally with aussi. See also: moi, toi, eux (elle, nous, vous and elles don’t have special emphatic forms).

Disclaimer: I don’t really speak French, but I know a lot of grammar.