I did not find a rule that says familiars can level up.
Warlocks can have more familiars than wizards (including imp) but I did not find anything about levelling up.
I guess the imp should have the same stats as the imps you fight in the intro.
My rangers spider did level up at some point, but its stats did not change.
I think the power of an animal companion should depend on the level of its owner.
Prof. Dr. Dr. Mad S. Tist
World leading expert of artificial stupidity.
Because there are too many people who work on artificial intelligence already
