when pressing left/right the character accelerate and moves, but the velocity does not decrease when releasing the direction button. i do not use the 2D game template. are there special peferences needed for the tielmap or st else that "move_and_slide()" function as intended (use friction to decrease speed...) ?
There's no friction when using the KinematicBody2D. You need to manually decrease the character's speed in your code.
Here, we don't make the character decelerate when releasing an input key: instead, we stop it instantly.
To make a character that decelerates smoothly, I recommend using steering behaviors. See this chapter to learn the basics: https://gdquest.mavenseed.com/lessons/introduction