If you’d like to remove characters from your Roster, go to the ‘Roster’ view and tap on the ‘Scrap’ button. You'll be brought to a screen where you can select between different rarity tabs. From there, you either handpick characters of that specific rarity to scrap, or tap on the ‘Select All’ button (Good characters only) to choose all available in that view. Once you have selected the characters you want to scrap, tap on the ‘Scrap’ button and confirm your choice.

Keep in mind that only fully promoted (maxed out) Awesome characters will show up in the Scrap view and are possible to scrap. While you cannot scrap fully promoted Ultra characters (at 5-Stars Ultra), you will be able to scrap any additional copies of the same Ultra character, as long as you already have this character maxed out.