I didn't know there was someone this cute in the world!
I can't believe someone this cute actually exists!
この場合、次のような言い方ができますよ。
ーI didn't know there was someone this cute in the world!
「こんな可愛い人が世の中にいるの!」
ーI can't believe someone this cute actually exists!
「こんな可愛い人が本当に存在しているの!」
someone this cute で「こんな可愛い人」と言えます。
ご参考まで!