Can I use any character for the name of my app? I'm preparing to publish an app I developed but the name I had in mind is already being used. To solve this I'd like to add a character/symbol (which relates to the name and gameplay) besides the name: ⟡. Is there a problem with doing this? Would this affect my app's visibility on the App Store.
According to the App Store Review Guidelines, names can be anything so long as they are no more than 30 characters, do not infringe on copyright, and are unique.

So, yes, you can use the ⟡ Unicode character in your app name so long as it follows the guidelines above and on the website.