You clearly haven't seen their history, have you?

I have. You make it seem like unawareness of their art style is of any relevance here, which it isn't. It doesn't matter what artworks they've created in the past.

It's not even new for this artist and they proceed to ignore everyone, purposely calling them "doodles".

Yep, I am not fond of that humble bragging either. But that's a different discussion.

You can have your freedom for your own ideas/designs, I see no problem with it.

That sentence should be the end of the paragraph. It's that: artists have complete freedom over their designs regardless of the source material. Period. Point. Over and out. The end.

But this certain character already have their existing design/ characteristics. They have already copy right.

Copyright is completely irrelevant here due to fair use. Derivative arts can be created and distributed so long as they are not commercialized. Copyright does NOT dictate that derivative arts must follow the source material exactly - and in fact, copyright generally serves the exact opposite cause, to prevent duplication of the source material without permission or fair use exemptions. For USA law, refer to section 17 U.S.C. § 107 of the Copyright Act.

Adding a feature they don't have, then Its no longer the character I know

Of course it is. You can still clearly recognize this is Yanfei. If you really want to use this argument, literally zero fanarts on this subreddit are of characters you know, because there will always be something different. Because it's art.

which I'm not fond of.

It isn't about you.