When Satsuki sets up his mom on a date with Kei's dad, Souta blabs to all his friends about it because he is so happy. This offends Kei who considers him an inconsiderate gossip. In attempting to insult Souta, she says things that he interprets to mean she doesn't like his mother.
In an attempt to spare his mother's feelings, Souta does not support the marriage and won't say way. Miyoko and Satsuki assume that he doesn't like Mr. Ichimura. Kei believes this is also the case until Souta explains to her what he thought she thought about his mom.
This is eventually resolved when he gets his help during a scavenger hunt when the thing you must look for is 'dad'.