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A Character in Swift represents a single extended grapheme cluster. An extended grapheme cluster is a sequence of one or more Unicode scalar values that, when rendered together, produce a single human-readable typographic character. Because Swift uses the same double-quote syntax for both strings and characters, a Character must be explicitly type-annotated during initialization; otherwise, the compiler infers a String. 
let explicitCharacter: Character = "A"
let inferredString = "A" // Type is String

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Extended Grapheme Clusters

A Character is not strictly bound to a single, fixed-width memory unit (like an 8-bit char in C). Instead, it encapsulates whatever Unicode scalars are necessary to form the visual character. This includes base characters combined with modifying marks, or multiple emojis joined by zero-width joiners (ZWJ). 
// Single Unicode scalar
let singleScalar: Character = "\u{E9}" // é

// Base scalar (e) + Combining acute accent (´)
let combinedScalar: Character = "\u{65}\u{301}" // é

// Multiple scalars joined by Zero-Width Joiners
let familyEmoji: Character = "\u{1F468}\u{200D}\u{1F469}\u{200D}\u{1F466}" // 👨‍👩‍👦
In all three examples above, the result is a single Character instance. When placed inside a String collection, each of these instances contributes exactly 1 to the string’s total count, despite the differing number of underlying Unicode scalars.

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Canonical Equivalence

Swift evaluates Character equality based on canonical equivalence. Two Character instances are considered equal if they represent the same linguistic meaning and visual appearance, regardless of whether they are composed of the exact same underlying Unicode scalars. 
let precomposed: Character = "\u{E9}"          // U+00E9
let decomposed: Character = "\u{65}\u{301}"    // U+0065 + U+0301

print(precomposed == decomposed) // true

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Memory and Indexing Implications

Because a Character can consist of an arbitrary number of Unicode scalars, its memory footprint is variable. Consequently, Swift strings (which are collections of Character instances) cannot be indexed using standard integer offsets in O(1)O(1) time. To determine the boundaries of a Character, Swift must iterate through the underlying Unicode scalars to evaluate where one extended grapheme cluster ends and the next begins.

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Introspection Properties

The Character type provides built-in properties to evaluate its Unicode classification and underlying scalar data without requiring manual bitwise operations or regex. 
let char: Character = "A"

let isASCII = char.isASCII               // true
let asciiVal = char.asciiValue           // Optional(65)
let isHex = char.isHexDigit              // true
let isLetter = char.isLetter             // true
let isWhitespace = char.isWhitespace     // false
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