Special character displays as square in TreeView in C# and VB.Net

There is a special character in my access database which is Ṣi. It displays as □i in Treeview. What could be the problem? Why is it not displaying as Ṣi in Treeviw?

Private Sub FillTreeView1a()
    If conn.State = ConnectionState.Open Then conn.Close()
    treeView1.Nodes.Clear()
    conn.Open()
    'get the data..
    sda = New OleDbDataAdapter("select Word from tbLexicon where LocalLanguage='" & cmbSelectLang.SelectedItem.ToString & "'", conn)
    'initialize the data set
    ds = New DataSet()
    'now fill the dataset with the result
    sda.Fill(ds, "Word")
    Dim I%, Category$
    'lets count all the rows we got in order to add them to the control
    'one by one...
    For I = 0 To ds.Tables("Word").Rows.Count - 1
        Category = ds.Tables("Word").Rows(I).Item("Word").ToString
        'we add the items one by one...
        treeView1.Nodes.Add(Category)

    Next

End Sub