This is almost as shitty a familiar as all those slimes from isekai series.
Also hadn't "zombie" been abandoned as stupid and wrong in favour of "thrall" before? I mean even though he is a ghoul now (and was a zombie when it said ghoul before).
I'm usually someone who hates familiars in fantasy anime because they are always so generic but I actually like the rat. Here's to hoping he doesn't evolve into some generic dragon or some bullshit. Though dragons in this manga do look unique, so maybe?
Also Edel with cloths on page 21 reminds me of another rat, can't remember off the top of my head unfortunately..
Lento should have told the rat to never again bite Vivi, or any other human friendly toward Lento, without a permission. He's used to schooling noob adventurers, yet somehow he's blind to such elementary things.