I'm the bad guy

Lilith

There are a few things that point to her.

Such as Alastor showing up right after Charlie asking her mother for help to them both being missing for 7 years. But this may be a little to on the nose. And after thinking about it, I'm not sure about it.

Lucifer

Ok, I can see where this comes from. But they way he interacts with Alastor tells me he doesn't have control over him. He's stronger then him. But Lucifer sees him as a threat to his relationship with Charlie.

Eve and root

I know nothing about these characters so I'm not going to speculate

Rosie

This is another one I can see why someone would think this. But I don't get this vibe from either of them.

I see them more as equal footing friend with mutual respect. It feels like he hangs around because he's just comfortable around her.

Niffty

On the surface this sounds like an odd one. But I have heard arguments for this and I have to say I love em.

One, she is summoned slightly different then husk. Two, Alastor seems to have a lot of respect for her.

Someone said that they could see Niffty going up to Alastor for an autograph only to be like, haha, now your my friend forever. But also wanting to seem like he's the one in charge.

This probably isn't the case. But I love Niffty and this person made it sound like a really cool idea.

zestial

Honestly, this is the one I can see being the most plausible. Alastor holds him in hight regard. And he's one of the oldest overlords meaning he lived threw Alastor's overtakeing of the other overlords.

I can the overlords at the time becoming corrupt and Zestial wanting to do something about it but not wanting to tarnish his own name.

What better way to do this then give a no body power to do just that. If Alastor gets taken out, not big deal. But if he win the other overlords are taken out and Zestial how secret control over Alastor.

And I see Alastor helping Charlie as doing the same thing. Everyone in Hell sees her as a joke, despite who her father is. So being there for her at her lowest. Gaining her trust and holding her power once she comes into her own.

So with context clues I think it's fairly obvious that Lilith is the one controlling Alastor.

But want I'm looking forward to see what he does when he's "free".

Will he continue to try to he charlie out of some found respect he gained because he's spend so much time with her. And in a way showing he was wrong about people being able to be redeemed.