In English, e can stand for a variety of different sounds, including none at all - words like "poke", "give", "lease", etc. have a silent e. However, because English borrows words from all kinds of different source languages, its pronunciation can be very inconsistent, and sometimes a word that might look to a native English-speaker like it has a silent e actually doesn't. Then the acute accent is often used to explicitly mark the e as being pronounced. The Japanese word "anime" is sometimes written "animé" in English, for instance, to prevent a reader from assuming it rhymes with "sublime", even though the Japanese characters in the word transliterate to simply a-ni-me.
Presumably, when bringing Pokémon overseas, the translators were concerned that people would read "Pokemon" and think it's pronounced "poke-mon" with a silent e, like the actual word "poke". The é simply makes it explicit that the e is pronounced.
é is not regarded as a separate letter in English; it's just a regular e that has a mark above it to communicate some extra information on pronunciation to the reader where it might otherwise be counterintuitive. It is not used in regular native words.
For more information, see the Wikipedia article on acute accents (you need to scroll a bit to get to the section on English).
