Hagrid, of course. He loves all kinds of strange and dangerous creatures. He would be the first to be intrigued by the Indominus Rex and would probably try to approach it to see if he could take care of it or at least understand it better.
Harry Potter himself could also be likely to interact with the Indominus Rex. Given his heroic nature, when the creature poses a threat to Hogwarts or the wizarding world, he would step up. He would use his skills and the help of his friends to try and figure out a way to deal with the situation involving the Indominus Rex.