From Muppet Wiki

Lou is a Fraggle who befriended Wembley in the first season Fraggle Rock episode "We Love You, Wembley." Wembley develops a crush on her, which Mokey tries to persuade him is a passionate love, enlisting the Trash Heap's help in bringing the two together.

After that episode, she more or less disappeared, although she was mentioned once or twice, and appeared in the background of some scenes.