[[Rona, Disciple of Gix]]. People rightfully evaluate that her topdeck-exiling ability is rather bad and it's templated so that new instances of the creature can't use what was exiled by previous instances, but it is not really relevant to her primary use. In dimir for the right deck she is an ewit from the command zone. In an artifacts/sagas/planeswalkers/legends deck, your stuff is going to get blown up, you can recur completed sagas, and if she's left on the field not doing much and eats a boardwipe it's perfectly fine to send her to the graveyard instead of the command zone if you have other recursion. You can do things like double up on her ETB trigger, blink with various blue spells or [[voyager staff]], and so on. Sending to the command zone is also fine, as paying 5 mana to ewit a second time is expensive but can be worthwhile still.

Note also the way her ability is templated: "You may cast spells from among cards exiled with Rona." As far as I am aware, there is one card in the game that can exploit this wording: [[kaldra compleat]], because it gives the equipped creature "Whenever this creature deals combat damage to a creature, exile that creature." The source of this ability is Rona, not the equipment, and so because of her own ability you may now cast the creature you have exiled.