These characters are 'free use'. To see the list, click on the title "Characters/Masks" at the top of the page. Mention them; spoof them; use them as NPCs, play them temporarily, if you like. Or adopt them. Sort of like a more fleshed-out version of Players Wanted, a bank of potentially-useful characters or random factors to use at a moment's notice.
If you want to add a Mask, simply put [[Characters/Masks]] somewhere on the relevant character page. If you arrange to permanently adopt one, remove that link from the page. If you want to use one, just do it. There are no permissions to be asked or granted. If you flesh one out, think about dropping a note to that effect on their page.