Red Goop

Red Goop will return to its owner's hand at the end of your turn unless the hand is full, in which case it will go to the Graveyard (similiar to Second Life).
The Speed Summon allows it to pick off weaker monsters immediately, and 22 life allows it to survive weaker ones (usually ability monsters).
In addition, every time you summon it, it "heals" back to its original 22 life. This can be comboed with cards such as Battery of Souls.
Combined with the fact that your opponent cannot attack it with their monsters, this makes Red Goop a very difficult monster to destroy.
It also has synergy with various effects such as Miasma Factory or Magic Staff, as their abilities do not affect Red Goop very much.
If you place it to the left of the slot with Soulsphere, its "return to hand" ability will trigger before the Soulsphere's ability, thus keeping it alive.
A 2 cost per play can be demanding if used as splash in a deck.
Goops are slightly colorblind, that is why Red Goop is orange, this is canon.