I do not recall any physical description of Peter. We have to visualize his appearance as best we can. But there is certainly description of Peter's actions, and that tells us a great deal about him. We know he is the oldest child in the family. We know he is very intelligent. We also know that he is sadistic and cunning by observing how he treats Ender and Valentine. The author shows Peter torturing and killing animals and manipulating the political world through his postings, which seems much like what we would call a blog today. In fact, Peter's personality and behavior is such a problem, he is not chosen to lead Earth in fighting the Buggers because of these problems.