Cyrillic kher writes the same fricative and takes the same numeral, six hundred, inherited from Greek chi where Glagolitic counts to it. Both names are opaque and both are recorded without an etymology.
The pair is the last letter at which the two numeral systems agree. At the letter that follows, the second sign for the vowel o, Glagolitic counts seven hundred while Cyrillic inherits eight hundred from Greek omega, and after that the two systems have nothing further in common. Glagolitic simply stops numbering: the Slavic letters that fill the end of its order carry no values at all. Cyrillic continues, giving numbers to two more Greek letters that Glagolitic does not have and fitting one Slavic affricate into a Greek slot that had fallen vacant.
That divergence at the end of the alphabet is the clearest structural difference between the two systems, and it is a difference of principle rather than of detail. One alphabet numbers what it has, in the order it has it. The other numbers what Greek numbered, and leaves the rest unnumbered. Which principle came first is part of the same argument as which script came first, and it is no better settled.
Both names in the pair are opaque, and the atlas records the fact rather than supplying an etymology for either. Heru and kher belong to the run of letter names at the end of the Slavonic alphabet for which no sense has been established, and an open question in the atlas states what a study of that whole set would have to do to close it.