The number of times you spend meditating unlocks the Celestian form. That means you have to spend as less time as possible bathing. When Nel appears, allow her to help you. There will be an event also when you go to sleep after meditating.
The demoness storyline is irrelevant to unlocking the form. However, at the current version, if you don't sign the contract, there may be some events you will be missing out.