My character needs to slide and not bounce off a slope.
The solutions I found here use a Reflection vector, but they make my character bouncy when they run downwards a slope.
var reflectVector = normal.clone().multiplyScalar(-this.velocity.clone().dot(normal)); this.velocity.addSelf(reflectVector); How can I make my character simply stick to the terrain when they walk? 
Eliminate the normal component from the old velocity using vector projection to get the desired velocity. Assuming normal is a unit vector, the formula is:
desiredVelocity = oldVelocity - dot(normal, oldVelocity) * normal; Applying gravity should accelerate the desired velocity, or if you just want constant speed, normalize it and scale as you wish.
You can also think this as a case of reflection. Reflection is oldVelocity - 2 * dot(normal, oldVelocity) * normal;. Here 2 can be replaced with (1 + e), where e is elasticy in the range [0, 1]. With 1 you get the perfect reflection and with 0 you get the formula above with your desired result. With a value between 0 and 1 you get small bounce.
here is pseudo code of one way it can be accomplished.
float speed = oldVelocity.Length(); tempVector = cross(oldVelocity, normal); desiredVelocity = cross(normal, tempVector); desiredVelocity.normalize(); desiredVelocity *= speed; 