I think it was because he was faintly attracted to Katha from the start seeing her work ethic and how dedicated she was. And in the end, she was single and pretty. He must have liked her a bit from the start and when she asked for money, while he wanted to test her as an expression of disappointment, a part of him also wanted her that way. If he hadn’t separated her from the others in his heart even without meaning to do so, he would have asked her why she needed it. He would have noticed her shaking. And even in the room, he would have pointed out her shaking. Viaan appears to be someone straight to the poibt and smart. Its just that he already had so many emotions towards her even without his knowledge. Lets imagine if after that night, Katha started looking like she was in love with him or admired him, where Viaan would have thrown any other girl out, he would have ‘managed’ her attention and I bet she would have been the first girl he would date because he did like her from the start. Viaan acts and treats her specially even without being conscious of it. And I think that even Kathaa knows it. If she is as observant as now, when Viaan acknowledges his love, she would be able to decipher something especially when she has been married before as is not a rookie. And even more, a small part of her isn’t resistant to him. Viaan and Katha are a treat to watch because their little everyday actions are the ones opening the other’s heart to them. Viaan is a short way from realising and I think when Katha does acknowledge hers, she would love him ALOT.