I think the object itself should be, as a mirror to the wildflower from summer, a flower of eternal ice.
But what does it do? I don't think they would go and do something malevolent just for fun. Or could, for that matter, since neither Inari nor Bobby are affiliated with the Faerie courts. However, I could see Mab plotting to get house Raith on her side, since she is collecting power left and right, and this would be a good way to get on Inari's big sisters good side. Maybe something that not quite counters Summer's gift, but complements it. Summer's gift is fertility, which will probably lead to a lot of offspring. Winter's gift could make those kids stronger, healthier, fitter. Maybe Mab is planning on breeding a new knight, or she wants to influence new emissaries from the start. She would well be devious and patient enough to do such a thing, and it would be easy enough to get the kids, since she could simply take back her gift and have them feel weak, so they'd come crawling to her for that strength back and maybe more.