Xi is distinct from the letter chi, which gave its form to the Latin letter X.
Greek
A joined variant of Ξ (New Athena Unicode font)
Both in classical Ancient Greek and in Modern Greek, the letter Ξ represents the consonant cluster /ks/. In some archaic local variants of the Greek alphabet, this letter was missing. Instead, especially in the dialects of most of the Greek mainland and Euboea, the cluster /ks/ was represented by Χ (which in classical Greek is chi, used for /kʰ/).
Because this variant of the Greek alphabet was used in Magna Graecia (the Greek colonies in Sicily and the southern part of the Italian peninsula), the Latin alphabet borrowed Χ rather than Ξ as the Latin letter that represented the /ks/ cluster that was also present in Latin.
Uppercase Ξ is used as an 'E' to stylise company names/logos like Razer (styled as RΛZΞR), Tesla (styled as TΞSLA), the presidential campaigns of Joe Biden (styled as BIDΞN), musician Banners (styled as BANNΞRS), and in South Korean boy group ZE:A's newest logo (styled as "ZΞA") (Compare: Heavy Metal umlaut; Faux Cyrillic)
U+039EΞGREEK CAPITAL LETTER XI (Ξ) (\Xi in TeX)
U+03BEξGREEK SMALL LETTER XI (ξ) (\xi in TeX)
U+2C9CⲜCOPTIC CAPITAL LETTER KSI
U+2C9DⲝCOPTIC SMALL LETTER KSI
The following characters are used only as mathematical symbols. Stylized Greek text should be encoded using the normal Greek letters, with markup and formatting to indicate text style:
U+1D6B5𝚵MATHEMATICAL BOLD CAPITAL XI
U+1D6CF𝛏MATHEMATICAL BOLD SMALL XI
U+1D6EF𝛯MATHEMATICAL ITALIC CAPITAL XI
U+1D709𝜉MATHEMATICAL ITALIC SMALL XI
U+1D729𝜩MATHEMATICAL BOLD ITALIC CAPITAL XI
U+1D743𝝃MATHEMATICAL BOLD ITALIC SMALL XI
U+1D763𝝣MATHEMATICAL SANS-SERIF BOLD CAPITAL XI
U+1D77D𝝽MATHEMATICAL SANS-SERIF BOLD SMALL XI
U+1D79D𝞝MATHEMATICAL SANS-SERIF BOLD ITALIC CAPITAL XI
U+1D7B7𝞷MATHEMATICAL SANS-SERIF BOLD ITALIC SMALL XI
