> naïve

There are actually a good number of words that historically used a diaeresis in English, especially up until about a century ago: noël, reëlect, reënter, Chloë, Zoë, noöne, preëmpt, daïs, coöperate, coöpt, coördinate, zoölogy...

The idea was to ensure that the second vowel would be seen as its own syllable.