Who is secret agent X-9 in Cat's Cradle and what is his significance?

In the novel Cat's Cradle by Kurt
Vonnegut, "Secret Agent X-9" is a nickname that is given to the character of Frank
Hoenniker, who is one of the sons of Felix Hoenniker: One of the forefathers of the
atomic bomb.

The main character of the novel, John, is
conducting an investigation for the purpose of writing a novel about the day when the
atomic bomb was dropped in Hiroshima, and so he goes after another one of the sons of
Hoenniker, Newt, to obtain more information.

However, the
attention shifts to Frank because of his odd personality, which is what earns him the
nickname of "Secret Agent X-9": He had a penchant for building things by himself, would
make bugs battle each other out, maintained silence at all times, and was basically a
loner.

Concisely, "Secret Agent X-9" is a nickname given to
Frank Hoenniker, one of the sons of the co-creator of the atomic bomb Felix Hoenniker,
and Frank earned such name for his odd and secretive behavior.