In elliptical constructions, some words are left out but understood. A pronoun in an elliptical construction should be in the case it would be in if the construction were complete.

His brother has always been more athletic than he [is].

Sometimes the case depends on the meaning intended.

Willie likes Lily more than she [likes Lily].

She is the subject of the implied clause she likes Lily.

Willie likes Lily more than [he likes] her.

Her is the object of the verb likes in the implied clause he likes her.