Characters obtained from gacha or events may not appear in your game immediately.
Please wait a little while before checking your characters again.
Make sure that you don't have a filter on, because then only characters matching the filter criteria will be displayed.
In the "Filter Characters" option, select "All" for "Group," "Type," and "Side Story" categories to display all characters you have.
If you have tried the above and still cannot see newly obtained characters, contact customer support from Menu > Support.