Complex sentence: The Stone was requested by Lupin's associate and on-off lover, Fujiko Mine, who, having agreed to obtain the Stone for a mysterious client, steals it from Lupin in Paris.
Split into simpler sentences: The Stone was requested by Lupin's associate, and on-off lover, Fujiko Mine. Fujiko has agreed to obtain the stone for a mysterious client, and she steals it from Lupin in Paris.