In the books
Harry Strickland is the current captain-general of the Golden Company and lord of the exiled House Strickland from Westeros. He looks little like a warrior, with a stout build, a big round head, mild grey eyes, and thinning grey hair that he brushes sideways to cover a bald spot. He often complains about blisters on his feet during long marches.
Harry actually used to be just the company paymaster (an occupation which seems more suitable for him than a commander), but after their previous leader died four years ago, he was made the new commander. He is not considered a very brave man, and many of his own captains thinks he is a coward. In many ways he is just a puppet for the true leader of the Golden Company, one of his lieutenants called "Griff".
Given that Harry is quite the opposite of what can be expected from a military commander - timid, insecure, hesitant and overcautious - it is unclear how come he was chosen to be the Captain-General of the company. Griff wonders about that: "This man is afraid to fight. How could they have chosen him to take the Blackheart’s place? He would wait until all seven hells were frozen if he could rather than risk another bout of blisters". Griff's son shares his father's negative opinion about Harry, referring to him as "an old maid".
House Strickland have been core members of the Golden Company for four generations, to its founding in the aftermath of the Blackfyre Rebellion, when surviving Blackfyre supporters fled into exile in the Free Cities. Among them was Harry's great-grandfather: Harry was born in exile and raised to serve in the Golden Company, as were his father and grandfather before him, making him a fourth-generation member of the sellsword company (which may explain why he rose to such a high rank despite his lack of martial prowess).
It is unknown what part of the Seven Kingdoms House Strickland hailed from. While most of the Golden Company are exiles or sons of exiles, due to the wandering nature of their army, Harry has picked up the nickname "Homeless Harry".