He was once a butler of the Zoldyck Family and the lover of Mitsuba.
13th Hunter Chairman Election arc
Hassam has never appeared in the story but is mentioned in a flashback detailing the discovery of the power within Alluka. When Mitsuba fails to fulfill Alluka's requests four times in a row as per the order of the child's mother forbidding the butler from heeding them, Mitsuba along with Hassam are both crushed to death as a consequence.
In Crunchyroll's translation of the 2011 anime, his name is "Hasamu" (identical to the rōmaji version of the character's name).