@wolfpuia the imp picks up item that you don't pick up, and converts them to a higher tier item. so if you have your pick up settings for magic +, the imp will pick up 5 normal items, and convert them to magic, with a small chance for a rare or higher item. the good thing about the imp is few items for you to pick up and have to sell all the time. the bad part is what ever the imp picks up, you can't sell it. in addition to the item being converted up, the imp will also sometimes pop out some orbs and gold.
so it is always 5 of one tier item to make the next tier item. the imp wont pick up Legend or higher items, but if you have your pick up setting on Legend +, all items the imp picks up will eventually end up as a Legend item, with a small chance for Crystal Legend. I am not sure, but I think there is a very small chance for it to be Eternal or Crystal Eternal. you just wont be able to sell your stuff for gold.
as far as what pet I like, imp. but using fairy at moment for the heal. I think slimes would be good for a 10% damage boost for builds that focus on 1 element. and hounds are good for farming builds, since they like digging up buried gold.