Leslie is one of the side characters of The Evil Within.
Before the events of The Evil Within, Leslie was a patient under care of Marcelo Jimenez. Before this, he was a patient of Valerio Jimenez's, and presumably part of Ruvik's plans. He was apparently rescued by the sudden appearance of Sebastian Castellanos and his co-workers, and is first seen during their escape from Beacon Mental Hospital.
He seems to be somewhat functioning despite his obvious mental illness, as he warns Sebastian about a trap in the second chapter, and is obviously capable of dodging them on his own.
Leslie can often be seen in the distance of levels, guiding the player to an end goal or warning them of traps and enemies. He seems to not be the target of Ruvik's creatures, as he is able to move relatively freely through the level, though this may be a gameplay choice rather than a narrative one.