Trix is a member of the Neverseen and a powerful Guster, who keeps his physical appearance hidden by using a Neverseen cloak. Keefe got the feeling that Trix and Umber frequently worked together, and if this is true, he most likely took her death pretty hard.
Although Trix hasn't made face-to-face contact with Sophie yet, we can assume that he has a powerful grasp on his his ability, as the Neverseen don't usually recruit someone into their order unless they have something useful to offer. This could also imply that he is in an advantageous position within the Lost Cities, possibly serving as a spy within the Nobility. Trix may also be a familiar character that we already know, such as Durand Redek, Marella Redek's father.
Nothing is known about Trix's past, real identity, physical appearance, or motives for joining the Neverseen. However, Trix has never been involved in a scene while Sophie is present, so this could change.
Although this was never actually stated in the books, in the section of Unlocked where Sophie writes down everything she knows about the various Neverseen members, she confirms that Trix was one of her kidnappers. It is unknown how she came across this information, as it was never mentioned in the previous books. There were only three main kidnappers (Brant, Gethen, and Alvar), but it's possible that Trix was also present at the Neverseen's Paris hideout.
Keefe was introduced to both Trix and Umber so he wouldn't be surprised about their involvement in Wylie's kidnapping. Neither Keefe nor Trix liked each other.
After Umber used shadows to open the Lodestar gateway, Trix, Alvar, and Ruy infiltrated the Silver Tower. While Trix immobilized Wylie with a large blast of wind, Alvar ripped Wylie's registry pendant off and Ruy trapped him in a force field. Trix and Umber most likely left after that, as they weren't needed for Wylie's subsequent interrogation and torture.
We know nothing about Trix's physical appearance, as he hasn't yet shown up in a scene where Sophie is present.
We know nothing about Trix's personality, as he hasn't yet to shown up in a scene where Sophie is present.
Umber (ally/possible significant other) - while infiltrating the Neverseen, Keefe told Sophie that he got the feeling Umber and Trix worked more closely together than some of the other members did.