The Scientist is an observer, a scholar. He or she can be specialized, as a botanist for example, or just a general scientist. This player can be adventuring in the wilderness or working in the cities, but the Scientist’s main goal is observation of life forms, culture, architecture, and behavior in the worlds he/she comes to.
Where an Explorer would be pushing into the wilderness to map it, the Scientist would be slowly moving along behind, making a catalog of new plants and animals, making sketches or taking screenshots. The Scientist longs for new discoveries to expand the existing world of knowledge, and also to be the first to name new species or elements. In fantasy worlds a Scientist can also study magic, if it is studied scientifically; likewise he or she might specialize in studying elementals in their natural habitat, or the behavior of dragons.
Any class can be a Scientist, but a character with Engineering skills will be best. Mages in fantasy settings make some of the best scientists, with their practices overlapping such fields as alchemy and astrology.