You may want to create a subRepeater.
<asp:Repeater ID="SubRepeater" runat="server" DataSource='<%# Eval("Fields") %>'>
<ItemTemplate>
<span><%# Eval("Name") %></span>
</ItemTemplate>
</asp:Repeater>
You can also cast your fields
<%# ((ArrayFields)Container.DataItem).Fields[0].Name %>
Finally you could do a little CSV Function and write out your fields with a function
<%# GetAsCsv(((ArrayFields)Container.DataItem).Fields) %>
public string GetAsCsv(IEnumerable<Fields> fields)
{
var builder = new StringBuilder();
foreach(var f in fields)
{
builder.Append(f);
builder.Append(",");
}
builder.Remove(builder.Length - 1);
return builder.ToString();
}