I’ve replaced the ethan model and added some stuff, however. is it still legal to use these assets in a commercial game?
The free assets Unity provides are designed as a starting point for you to build on top of, which is exactly what you are doing, so you are fine.
As for other free assets…
Some developers may include a note asking you to let them know if you have used their assets so they can check it out in a full production, but nothing legally binding.