On a String , I can use utf8 and count to get the number of bytes required to encode the String with UTF-8 encoding:

"a".utf8.count // 1 "チャオ".utf8.count // 9 "チ".utf8.count // 3

However, I don't see an equivalent method on a single Character value. To get the number of bytes required to encode a character in the string to UTF-8, I could iterate through the string by character, convert the Character to a String, and get the utf8.count of that String:

"チャオ".characters.forEach({print(String($0).utf8.count)}) // 3, 3, 3