The slime companion(s) are found in level 1 behind a couple doors that require a lvl 1 key card, which can also be found in level 1 (it's carried by a minimech near the purple control terminal). The gif on this itch page of Mia going through a door is actually the first of those doors you need to go through (there IS also another door that will lead to this area elsewhere in level 1, but you'll still need the lvl 1 key card in order to get through the final door before the companion).
Glad to hear the wall jumping works well! I think I underestimated how difficult it would be to learn for players since I was the one who made it, but I'm happy to hear that once it is learned, it handles well.
There's still plenty more game yet to come so stay tuned :)