There's one more character that we shouldn't forget.
There's one important character we should be sure to remember.
There's one more character that we shouldn't forget. のように one more character と言うと、他にも忘れてはならない登場人物がいることになります。
ただ単に「忘れてはならない登場人物がいます」と言うのなら、次のように言うと良いでしょう。
ーThere's one more character that we shouldn't forget.
ーThere's one important character we should be sure to remember.
「必ず覚えておくべき大切な登場人物がいます」
ご参考まで！
There is one important character that you can't forget.
There's one really important character that you shouldn't forget.
ご質問ありがとうございます。
・「There is one important character that you can't forget.」
「There's one really important character that you shouldn't forget.」
（意味)絶対に忘れてはならない[登場人物](https://eikaiwa.dmm.com/uknow/questions/55898/)がいます。
<例文>There's one really important character that you shouldn't forget. Can you guess who I'm talking about?
<訳>絶対に忘れてはならない登場人物がいます。 誰のことかわかる？
参考になれば幸いです。
カナダ 
アメリカ合衆国 
