Somewhere in the third part of the game, there is a cutscene where Medusa would leave the group and attack. However, if you just grab the red cardkey and head back to the museum without triggering that betrayal cutscene, you can still take him outside the building. Am I missing out on anything? Perhaps some bugs or extra dialogue?

Can't remember well, but I think at the top of the museum Medusa will go crazy and there is no way to avoid that. Anyways... who wants to keep him? I did not find him quite useful...

I know this is a relatively old post but...

I like Medusa and find him to be effective (better than Slavsky anyway). One of his attacks can attack every square around him, and tranquilizes to boot. If you're clever, you can hit multiple enemies with his other attack, its range is like the flamethrower's. Perhaps the only disadvantage of him is that he can only move 3 squares per turn, i.e. he's slow.

Just my 0.02$.

Vorax

Sorry for Necroposting, but I have another tip and I didn't want to start a new thread.
It's a spoiler too, so beware.

Many people have really big problem with one of the first bosses in game - the mother with her childrens. When you kill one of her offspring, she starts going medieval on your sorry hides. The actual trick is to NOT kill the children before you kill her - they're pretty harmless (compared to their mom, anyway), and she don't use her venom attack that way, making her pretty docile opponent.

Actually, there is much more funny tactics that you can discover in this game. The actual puzzles are not those about using item to gain something, as they're pretty obvious, but those that involves using your enviroment during combat for your advantage. Remember, you can push boxes, sometimes blocking your enemies and rendering them defenseles.

Yeah you can just don't trigger the betrayal cutscene , he will come with you up to the secret laboratory.

Every optional character has its own dialogue right before entering the final stage. They don't differ much though.