This isn't explained in game, so this is an important tip for new players.
You likely have noticed that the Crawler you select determines your starting deck. What isn't as obvious is that it also determines your starting HP and Mana.
When you unlock additional Crawler slots from the shop, your secondary Crawlers only add their personal card and weapon to the deck. They don't impact your stats. As far as I can tell, Outhouse Crawlers only add their personal card to your deck.
This means that the same combination of Crawlers can play dramatically differently, depending on the order.
Endgame Spoiler:
>!This is especially noticeable with MissingNO, since he starts with an obscene amount of HP and Mana in addition to a large deck. If he's in a secondary slot, he's much more reasonable. Still BROKEN, but... Not AS broken. !<