add the correct encoding on your web.config.
<globalization requestEncoding="utf-8" responseEncoding="utf-8" />
Its also depend how you save your asp.net file. If you save it as code page 1253 (that is the Greek for alpha -> α) or UTF-8. Its better to save it as utf-8 and show it as utf-8
about globalization https://docs.microsoft.com/en-us/previous-versions/dotnet/netframework-4.0/hy4kkhe0(v=vs.100)
for html files
you declare the charset right after the header as:
<head> <meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
<meta http-equiv="Content-Type" content="text/html; charset=utf-8"/>