Alby, the First-in-Command
The First-In-Command was the leader among the Gladers. The First-In-Command was usually the person who had been in the Glade for the longest. The position seemed to come with more responsibility than power. When decisions needed to be made, the First-in-Command would call a Gathering and led the discussion. There seemed to be two instances of teens with the role of First-in-Command in the Books. Alby was the leader of the Gladers, being new to the job when Thomas first arrived on the Glade, as he mentioned their former leader, Nick, had recently died before Thomas arrived. Newt was his Second-in-Command and took over the role of leader when Alby was unavailable. It seemed that Harriet had the same role as Alby within Group B, with Sonya presumably being he Second-In-Command.
In The Scorch Trials Official Graphic Novel Prelude, which is canon within the Film-Universe, Ximena had died similarly to Nick, which was how Harriet came to be the leader with Sonya as her Second-In-Command.