ZATR (Tak) is the romantic pairing of Zim and Tak.
Tak´s and Zim´s relation ship is only seen in the episode: Tak: The Hideous New Girl. In the episode we can see Skool children celebrating Valentine´s Day. After Tak joining the classroom in the celebration, she reads a poem to Zim much to his surprisement.
Jhonen, the series' creator, hates the pairing (along with ZADR) and has stated countless times that Tak is Zim's enemy, not his love interest. (Though he has left the door wide open for Zim and Gaz's love and Dib and Tak's love to bloom).
After classes, Zim concludes that the new girl is in love with him so he takes this chance to "study" human feelings but only swearing he will at the end destroy her.
Despite all his efforts Zim is not well accepted by human Tak, but he is surprised when he finds out that Tak is a female Irken and she wants to steal his mission from him so she could prove herself as an Invader to the Tallest.After Tak invading his base, Zim goes to Dib´s house for help. This may be another hint, because Dib and Zim were discusing about Tak and Dib mentions Zim is just jealous about Tak thus he denies it, Zim assures Tak is pretty good but not better than him.