Joker is shown to be a person Luea is keeping a secret about, helping her spawn beasts. This anonymous person is shown in Lady Jewelpet Episode 28, when Luea turns a straw-man doll into a monster. He probably seems to be against Jewelland instead of helping Lillian, as shown in Lady Jewelpet Episode 43 when he uses the Final Wand to take out Luea, but Lillian sacrifices and vanishes. A plot twist centering around this character is revealed episode 46, when Miura discovers that the Joker is actually Lady Rector.
Joker is the person behind the beasts, as shown in Lady Jewelpet Episode 27.