[DefaultValue("")]
public string FontFamily { get; set; }

<DefaultValue("")>
Public Property FontFamily As String

<dx:ASPxRichEdit ID="richEdit" runat="server" WorkDirectory="~\App_Data\WorkDirectory">
  <Settings>
    <Fonts DefaultFolder="fontsFolder/">
      <FontCollection>
        <dx:ASPxRichEditFont FontFamily="Calibri" Name="Calibri"
          RegularFontUri="CalibriFont/calibri.ttf"
          BoldFontUri="CalibriFont/calibrib.ttf"
          ItalicFontUri="CalibriFont/calibrii.ttf"
          BoldItalicFontUri="CalibriFont/calibriz.ttf" />
        <dx:ASPxRichEditFont FontFamily="Comic" Name="Comic" />
        <dx:ASPxRichEditFont FontFamily="Lemonada" Name="Lemonada" UseGoogleFonts="true" />
        <dx:ASPxRichEditFont FontFamily="Roboto Mono" Name="Roboto Mono" UseGoogleFonts="true" />
      </FontCollection>
      <MappingSettings DefaultFontName="Calibri">
        <Rules>
          <dx:ASPxRichEditFontMappingRule SourceFontFamily="Arial" DestinationFontName="Comic" />
          <dx:ASPxRichEditFontMappingRule SourceFontFamily="Times" DestinationFontName="Roboto Mono" />
        </Rules>
      </MappingSettings>
    </Fonts>
  </Settings>
</dx:ASPxRichEdit>