A Denobulan scientist of the 24th century, one of those who hid the Progenitors' technology, who built weather towers to save the people of Halem'no and left a clue inside one.
Hitoroshi Kreel is discussed in Whistlespeak. An expert in weather modification, he visited the dying pre-warp world of Halem'no and built five towers disguised as mountains to make rain and hold back the dust storms.
By 3191 only one tower still works, and the people living in its shelter have built a religion around it. When Burnham and Tilly arrive in disguise they find that the tower's rain chamber is being used for ritual sacrifice.
Kreel's clue lies inside the structure. Burnham reads his choice of hiding place as a warning that powerful technology demands care for the cultures it touches, which she must weigh against her own decision to reveal herself to the Halem'nites.