comparing object attribute in one array to an object attribute in another array

Answer 1

If looking up character objects by ID is a common operation, you should create a mapping from id's to character objects, at which point doing the lookup from your second array will be trivial:

NSMutableDictionary* charactersById = [NSMutableDictionary dictionary];
for (Character* character in characters) {
    [charactersById setObject:character forKey:[character IDE]];
}

(Note that I've made your class upper-case for the example code; you should do the same in your code since writing code that ignores standard language conventions is a bad idea in any language; it significantly reduces the readability of your code.)