Learn English – Why is “Pokémon” written with an accent

The mark in question is an acute accent mark and is absolutely intended to mimic the native Japanese pronunciation, which itself is based on the English words "pocket monster".

Because of English orthography, there is considerable ambiguity surrounding the pronunciation of the character "e". (Compare the way you pronounce the "e" in "pocket" with the way you pronounce the "e" in "peel" to see this for yourself.)

However, in Japanese, the orthography and pronunciation of "e" sounds presents no ambiguity. To accurately transcribe the "e" sound found in the English word "pocket", only the character ケ, which is pronounced as /kɛ/, can be used.

Therefore, to indicate the correct pronunciation in orthographically-complex, ambiguous English, the acute accent was used. The reason for this decision was because although no native English words use the acute accent mark, most English speakers, and many Japanese speakers, are familiar with the way in which an acute accent mark modifies pronunciation, due to the large number of existing French loanwords in both English and Japanese--entrée/アントレ, café/カフェ, and élite/エリート are familiar examples of this pattern.

Source: I am a Japanese speaker and have worked at Nintendo as a translator.