Spoiler for Hidden:
Lucky Slime7

| 7

7 | 7
Loses -1|-1 and 1 cost each turn. At 1 | 1, becomes 7 | 7 with cost 7 again.
Upgraded text: Loses -1|-1 and 1 cost each turn. At 5 | 5, becomes 7 | 7 with cost 7 again.
dragtom - A pretty good interpretation but not quite what I had in mind.
Discord - Pretty much what I had with slightly different stats, though it lacks cost efficiency.
Elk - Interesting take on it but I don't quite understand that last part - is the swap to +2|+2 boosting permanent?
Frozengaia - Spot on in stats but wrong element. Upgraded has a possibly better implementation than what I had in mind for cost efficiency, though.
1st) Frozen - While the element's wrong you did get my aim for cost efficiency right, which I feel is more important.
2nd) Discord
3rd) dragtom