Код

HTML

<div class="player">
			<!-- Персонаж -->
		</div>

CSS

.player {
	border: 1px solid black;
	width: 6%;
	height: 17.3%;
	position: absolute;
	bottom: 30%;
	left: 68%;
	transition: left 0.5s linear;
}

JavaScript

document.onkeydown = function(e) { 

	if (e.keyCode == 37) {
		posX = document.querySelector('.player').offsetLeft;
		xn = posX - 50; 
		var timerId = setTimeout(function() {
			document.querySelector('.player').style.left = xn + 'px';
		}, 0);
	}

}

document.onkeyup = function(e) {

	if (e.keyCode == 37) {
		clearTimeout(timerId);
	}

}