Think about it from his point of view. Before he managed to grew suspicious of Zhenya enough to contact his headquarters and find more information about him, he got cut off… he had no way of finding information about him and ever since the bombing of his apartment in the beginning he was actually relying on Zhenya with almost every basic need. There are hints throughout the story that indicated he was suspicious of Zhenya and kept his guard (even if he slipped sometimes), but altogether, he had no solid evidence that Zhenya is not who he claims to be. Of course, for us readers it is evident from the beginning who Zhenya truly is, but from Taekjoo’s point of view, it was confusing at best.
Ngl why is taekjoo kind of a shit spy or am I just tweaking