WMG / Wintersmith

There are actually two Winter elementals and two Summer elementals.

Because the Discworld isn't a sphere, its seasons aren't derived from planetary tilt; rather, the parts of the rotating Disc that are nearest to the orbiting sun's overland path experience summer, and the parts that are perpendicular to that path have winter. That means that there's two areas of Disc that undergo a winter, and two that have summer, at any given time, as the seasons progress Widdershins around the spinning world.

The young-seeming and (initially) immaterial Wintersmith whom Tiffany Aching meets has a counterpart: the elderly-looking, icicle-bearded Jack Frost, who'd previously appeared in Hogfather. It's the latter whom the illustrator had been thinking of, when he drew the Spirit of Winter for Miss Treason's book of mythology, not realizing that there were two of them. Likewise, the Lady of Flowers and Summer Lady aren't actually the same person, although most Discworld mortals tend to conflate them.
  • This seems almost completely confirmed by early Word of God, as the Discworld has always canonically had eight seasons (remember the importance of eight on Discworld?).