I am a beginner to php so correct me if I'm wrong, but I think if you're using if-else statements then both should be within the same php tag. Since you are trying to set a value "Des" or "Dave", you can use a single php tag in which you could create a php variable to store the value and then assign it after closing the php tag. This could also help avoid the spagetti code situation as explained by Jodunno. So to adjust the code you provided:
<?php
if ($_SESSION["DE_Retain"] == 1) {
$value = "Des";
} else {
$value = "Dave";
}
?>
<input type="text" maxlength="32" size="42" name="<?=$Comment_Name ?>" value="<?=$value?>"> Max 32 Characters<br><br>
See if this works and if it does, do let me know as well.