Since I don't think this question should be closed with no answers, here's a brief summary: most of the research (never mind the popsci articles, which far outnumber actual research on this) seem to focus on the profile/personality of intimate partners who "fall" for narcissists.
Haslam and Montrose (2015) after surveying a British sample of young females (18-28 years old) say that
Females value different traits in short-term and long-term partners. [...]
Females with greater mating experience and those desiring marriage were more attracted to the narcissistic male personality. The narcissistic personality, whilst having many negative qualities, possesses qualities associated with status and resource provision. These traits are desirable in short and long-term mating contexts. Despite future long-term mating desires which are unlikely to be achieved with a narcissistic male and possession of substantial mate sampling experience, females view the narcissistic male as a suitable partner: a testament to the success of the narcissistic personality in facilitating short-term mating.
While this doesn't say much about the personality of the narcissists' potential mates, it does say something about their goals (aspriging for status and resource provision).
A study on speed dating by Jauk et al. (2016) found that
Across both sexes, narcissism was positively associated with mate appeal for short- and long-term relationships. Further analyses indicated that these associations were due to the shared variance among narcissism and extraversion in men and narcissism and physical attractiveness in women, respectively. In women, psychopathy was also positively associated with mate appeal for short-term relationships. Regarding mating preferences, narcissism was found to involve greater choosiness in the rating of others' mate appeal (but not actual choices) in men, while psychopathy was associated with greater openness towards short-term relationships in women.
Again not much is said directly about the personality of the partner, but something is said about what they saw valuable in narcissists (extraversion, looks).
On non-intimate but long-term relationships, the OP himself found Maaß et al. (2016):
Results showed that the friends' similarity in narcissism significantly predicted similarity in all Big Five domains. For the general Big Five similarity as well as extraversion, the effect of narcissism similarity was stronger for male than female or mixed friends. Similarity in psychopathy and Machiavellianism significantly predicted all domains except for openness and extraversion, respectively.
And that's basically summarized in the paper's title "Narcissists of a Feather Flock Together".