The suffix -EVER is added to certain relative words WHO / WHOSE / WHOM / WHICH / WHERE / WHAT to express meanings such as 'it doesn't matter what', 'all', or 'every':
Whichever decision you make, I'm sure it will be the right one. It doesn't matter which decision you make, I'm sure it will be the right one.
Whoever stole my mug should bring it back immediately. The person who stole my mug should bring it back immediately, and I don't care who this person is.
The director will cast whomever he finds suitable. The director will cast the person he finds suitable.
Whatever he asks for, you should give it to him. No matter what he asks for, you should give it to him.
Whenever she speaks in public she makes a fool of herself. Every time she speaks in public she makes a fool of herself.
Wherever she goes, she meets him. It doesn't matter where she goes, she always meets him.