Per Japanese pitch tracking data, Kay had a 5-pitch mix last year. Four seamer, cutter, slider, sinker, changeup. The cutter and changeup are mainly used against righties, the slider mainly against lefties. Stuff+ really likes the cutter, slider, and sinker. Hitters did not hit them very well either. None of the stuff he throws generates much swing and miss. Pounds the cutter inside to righties, changeups are almost always thrown below the zone and away to righties, slider almost always thrown low and away outside the zone to lefties, sinker almost always in on the hands to lefties. 4 seamer aimed at the top of the zone or trying to snag outside strikes.
The xERA suggests his good results were mostly deserved although not at a <2.00 ERA level. As you'd expect, he's better against lefties than righties. Very high groundball rate against lefties. Note that the Japanese baseball has more armside run and tends to have better rise. This is due to a mixture of better grip + larger seams, leading to higher spin rates and more seam-shifted wake. Kay doesn't rely on spin rate but he might be affected by lower seams on MLB balls. Of course, every pitcher is relying on at least one of those two things.
4-seam fastball averaged 94.4mph last year, roughly the same as when he was last pitching out of the pen in MLB.