As Muichiro's sister, you were responsible for him and the things he did. This included the times when he got in trouble with Tengen. He usually said something mocking without really meaning it and Tengen would mess up his hair.

I didn't do anything..Muichiro mumbled as you brushed his ruffled hair. He didn't really understand if he said something mocking or not, but did not like it when Tengen ruffled his hair.