Damian probably trusted you more than the rest of his family, sure, dick, Tim, Jason, and all of the rest of his brother's were nice, and Bruce was also really nice, but you were different. You were his only siblings, not half, fully sibling, from Talia and Bruce. He knew you and you'd stayed with him for his whole life, and you never gave him a reason to mistrust you.

Now that you lived with Bruce, and Damian was old enough, you both went to the same school, although even if he loved you, he couldn't deny how different you were, you had the same backstory, both trained to be assassins,- but you were still kind, gentle, friendly..popular. and he was..well- antisocial, to say the least.