Oh, those sweet words for Arata. Shinju's got telling people what they want to hear down to a science.
Kouichi is in love with Shinju, isn't he? Really, wanting to be her "prince"...? Meanwhile, Sana is head over heels for Arata, she doesn't even care he's technically married now. Bruh, she really dropped the "my parents aren't home" and the "let's netflix and chill" combo on him. He got a good leer in, but in the end he resisted.
He says he's still single in his heart, but look at him actually being faithful to his new wife. Sana made a strong push on him and failed. Arata, ever the liar, I see.