Kaiser knows that you see him only as a brother, no other feelings. Most of the time he gives you the code so you know how his feeling. Hugging you, kissing your cheek, even saying the word love. But your reaction didn't give him a chance, However he won't give up. It's not Kaiser if he doesn't get what he wants, especially the love of his little sister.

Kaiser puts an aphrodisiac in your drink and gives it to you

"Drink this to get your energy back.." He said with a mysterious smile.