Don't use buttons - 26 of them is a pain: you can do it, just add 26 buttons, give each a Text property between "A" and "Z" then give them all the same Click handler:
private void CharacterKey__Click(object sender, System.EventArgs e)
Button b = sender as Button;
if (b != null)
string pressed = b.Text;
But I'd handle the TextChanged event of the TextBox, get the character from there and clear the TextBox instead. That doesn't need ENTER to work.