i need to create an entity that act exactly like the player, to follow it, rotates along it, etc... and it has to be always behind the player even if the player faces in a different direction.

Did you so a search in forum ? resourche ? aum's ?

if so then you missed this one :

[codefunction player_follower();
action players_code() // attach this action to your player model
{
var movement_speed = 20;
VECTOR temp;
set (my, INVISIBLE);
player = my;
ent_create("guard.mdl", nullvector, player_follower);
while (1)
{
my.pan -= 7 * mouse_force.x * time_step;
vec_set (temp.x, my.x);
temp.z -= 10000;
temp.z = 0;
temp.x = movement_speed * (key_w - key_s) * time_step;
temp.y = movement_speed * (key_a - key_d) * 0.6 * time_step;
c_move (my, temp.x, nullvector, IGNORE_PASSABLE | GLIDE);
camera.x = my.x;
camera.y = my.y;
camera.z = my.z + 50;
camera.pan = my.pan;
camera.tilt += 5 * mouse_force.y * time_step;
wait (1);
}
}
function player_follower()
{
var walk_percentage;
while (1)
{
// play with the numerical values; they give the xyz offset of the follower in relation with the player
vec_set(my.x, vector(150, -60, 0));
vec_rotate(my.x, you.pan);
vec_add(my.x, you.x);
my.pan = you.pan;
ent_animate(my, "walk", walk_percentage, ANM_CYCLE); // play the "stand" aka idle animation
walk_percentage += 5 * (key_w - key_s) * time_step; // increase walk_percentage only if the player is moving
wait (1);
}
}
][/code]]


From here you have to study this code make changes and
make it do what you really want

good luck

Use vec_set, vec_rotate, vec_add for the position (negative x-distance, rotate by player pan), then vec_set for the angles, ent_animate the entity the same way (if you have complex animations) or use the frame and next_frame parameters to copy simple animations.

this is my code, but the entity doesnt follow the player when it walks, and it doesnt always creates behind, if the player faces in the opositive diction the entity created ahead.

ent_create("newentity.mdl",nullvector,entity);


function entity()
{
while (my)
{
vec_set(my.x, vector(0, -4, 3));
vec_rotate(my.x, player.pan);
vec_add(my.x, player.x);
my.pan = player.pan;
wait (-1);
ent_remove(my);
}
}