Henry is a former boxer and trainer who owns the Harbor Health Club, an upscale gym where Spenser and Hawk train. The Harbor Health Club started out as a boxing gym back when the waterfront was still an economically depressed area. Throughout the series, as the waterfront becomes more and more gentrified, the Harbor Health Club does as well. By Valediction, the gym has become sufficiently upscale that Hawk complains to Henry. Through the Health Club's various incarnations, Henry keeps a small area with punching bags for Spenser and Hawk to train with, and allows them to work out for free. The Health Club serves as a point of contact for Hawk, whose home is never revealed or discussed in any of the books.
Spenser describes Henry as "a pint and a half of muscle stuffed into a pint-sized shirt". Henry's height is alternately given as 5'4" and 5'6" - he remains in excellent shape throughout the series. In Henry's youth he famously fought (and lost to) real-life boxer Willie Pep, who held the World Featherweight championship twice between the years of 1947 and 1950.
Henry first appears in the second Spenser book (God Save the Child) and appears or is referenced in every book afterwards. Not much is known about Henry's background, but he has no qualms about helping Spenser and Hawk with some dubiously legal activities such as engineering a jailbreak (A Catskill Eagle) or storing some cocaine which Spenser steals from a drug lord (Pale Kings and Princes). He occasionally offers armed backup to Spenser for various dangerous activities, though the offer is never accepted.
In Ace Atkins' novel Wonderland, it is revealed that Henry lives in a condominium on Revere Beach, and was in a relationship for a decade with a woman who has since died.