In the beginning of the series, she constantly hugs and kisses Ataru whether he wants it or not, often infuriating Shinobu (who still initially had feelings for Ataru, though she eventually gave up on him) and the rest of the class.
Lum's relationship with Shinobu was initially very stormy, as Lum considered Shinobu to be Ataru's "mistress" and went to extraordinary lengths to disrupt the relationship between Ataru and Shinobu (including using her UFOs to jam Ataru and Shinobu's telephone calls and claiming that she was pregnant with Ataru's child).