It will most certainly NOT be random, don't worry lol
The idea behind Imitator is that they would be able to mimic another subclass by gaining access to its skills, but they wouldn't get the stat bonuses that the subclass gives you. For example, an Imitator could get the Fury skill from the Juggernaut class, but they wouldn't get the Attack increase that comes with the Wrath tree.
Essentially they'd be sacrificing power for versatility, being able to use skills from other Sins but not as effectively. This would also allow for more freedom in party composition, since you could build your imp around your units rather than the other way around.