Well. Set the movetype of the entity to MOVETYPE_FOLLOW. and set aiment to the entity you want it to follow.
And if your trying to replace the player model in esf this method does work. Just is very ugly in adv melee. I have yet to find a legit way of changing the player models. So I use this method.
You also gotta hide the player by making them render invisible. ( DO NOT USE set_user_visibility ).