Trillian is a torivor who came from another world and into the Outskirts, where he was taken by powerful shapers and trapped under a castle known as the Lost Palace. Trillian, as a torivor, has incredible shaping abilities, although he is unable to extend his powers outside of the Lost Palace in Elloweer, where he is trapped.
Cole and his friends went in search of him as he was thought to be keeping Mira's sister Honor captive. They followed a bright red road named the Red Road to his palace. Trillian met up with Jace and Cole in a dream, as torivors were known to do in Beyonders. He told them that if they wanted Honor the two of them along with Mira would have to complete a challenge, if they failed he would keep them. The challenge involved going through three different scenarios in all of which, Honor would be there but placed under heavy seemings, the trio had to figure out who she was, touch her and yell out "Honor!" In the third scenario Mira figured out who she was and freed her. Trillian guided them to the Grand Shaper of Elloweer, Callista who lived in the Fog Lake.
When Trillian appears to Cole, he seems to be a human, a mix of a races, with Asian most prominent.
Although Trillian appears to Jace and Cole in a dream he is not in his true form. He is most likely to look like on of the Torivors from Beyonders " It was said that if Death took a form, it would look like a Torivor. Their shape is, to a certain degree, human. Their whole form is pitch black, even in the sunlight. Their eyes are completely black, with no whites or irises. They move quickly and silently, making them great spies." (Beyonders Wiki)