While clearly getting the permission of the previous dev is the best route to take... as long as you are not profiting off of it, you can pretty much do what you want. If the character models are available there is no reason you can't use them.
As an example, when the Mandalorian came out and baby Yoda was revealed Disney forced people that posted screen shots (even memes) to take them down*. However, for artist who drew, painted, crafted copies of baby yoda and posted them, Disney could not do anything about it.
*While some of the usage could have been considered "fair use", I am not aware of any person going against Disney's order to take them down. What average person would want to go to court against Disney?
*Disclaimer: I am not a lawyer, this is not legal advice, just my opinion on events I have witnessed in life