Actually, that does beg the question - does a character appearance make the novel a part on an overall arc or series, or does that novel have to include a plot point or a theme to be considered that?
Intersting thought.
I remember some early novels that were loosly tied together by the inclusion of the security chief 'Thomson'
J.M. Dillard invented the character for Mindshadow with following use in Demons/Bloodthirst/The Lost Years.
The character was also used by Gene DeWeese in 'Chain of Attack'
I think Duane may have used Thomson in some of her novels? Which would pull all above novels into the 'Duaneverse' Sort of.