You know the Crystal Gems? If you do, Koin's weakness is pretty similar. She is a slime with a powerful amethyst core that generates her mass, it is mainly used for regeneration once the body is completely formed (so the perfect weapon to hunt her down is a spear you can throw).

But weight! She is special, of course! Unlike other slimes, Koin is enormous (single-story house-sized)! "So why can't I see it in the drawing?" Because she is scattered about like an agar.io bacteria! That's her strategy, don't spoil it.

The thing is, she likes money, a lot. To the point that she stops everything she is doing to go after the shiny valuable object in your pocket.

Extra ability: She absorbs feelings of greedy from people, which helps with her regeneration.

These sprites are for the "game", aren't they?

yeeeeah riiiight!

In the "game", Koin would be the magician of the group, with great powers and low physical attack. She isn't fragile though, having a lot of HP.

As I said in an article that not even I remember: Koin has a skill called "Trade II". I call it like that because it is better than Helen's Trade I that affects both her and a chosen party member, turning them into a single super fighter. The method she uses is to engulf you entirely and control your body.