The ü is typical for an UTF-8 originated ü being incorrectly encoded as ISO-8859-1 instead of UTF-8. Here's a programmatic evidence:
System.out.println(new String("ü".getBytes("UTF-8"), "ISO-8859-1")); // ü
Since you mention that the very same character from the properties file works fine in some JSP's, but not in other JSP's, then it means that the browser is by those JSP's not correctly been instructed to use UTF-8 to display the characters returned by the server.
This instruction happens in the HTTP Content-Type header. Using any HTTP header debugging tool, you must be able to figure the returned header. One of the popular tools is Firebug.

Note the presence of charset=utf-8.
Usually, in JSP this is achieved by simply placing the following line in top of the JSP file:
<%@ page pageEncoding="UTF-8" %>
See also: