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You can choose to unlock and get the avatars at level 20, but there is a small price to pay for doing that.

If you are successful in beating the avatars, then of course you can use them. However, in order to fight them again in the future, you will not be able to warp to the cloisters, you will need to walk to the cloisters and commence battle there(and believe me, walking in the den of rancor is a mission in itself :P )

I didn't get my fenny until 72 and I still haven't gotten diabolos (yeah i'm lazy i know) yet, but don't worry! :)

If you have any other questions, feel free to ask! I'm sure there are a lot of other (and probably better) smns than I than can give you some really awesome advice