I feel like Umi is not a bad dude.
He pretty much knew what Chizuru was thinking when she rejected his advance and indirectly said she liked Kazuya.
Besides, I never took the phrase "I don't not like him" literally.
The meaning behind those words is actually someone struggling to admit to his or her own feeling.
I think Chizuru had started developing feelings for Kazuya ever since he saved her at the beach.
But since he's her client, Kazuya was still trying to get back with Mami, and she wanted to keep being a professional, she's been bottling up her feelings towards him.
Her saying "I don't not like him" was basically like you getting caught lying and you don't want to admit it, but you also suck at lying so you have cognitive dissonance and say something contradicting.