French is a small-time career criminal, and is characterized as unkempt, aggressive, and witless. According to Irish, he met French along with Alwyn Lloyd (a.k.a. "Welsh") on the boat to America, and were 'thick as thieves', literally.
French and his accomplice, Welsh, are in the middle of interrogating Irish by drowning him in a water trough. They accuse Irish of stealing their horses, and demand retribution. Irish claims he is innocent and that French was the one who stole the horses, although this is never proven. The three are interrupted by John Marston, sent by Nigel West Dickens to inquire with Irish about weaponry to assault Fort Mercer. Marston is forced to kill both Welsh and French in self defense.
Just like Welsh and Irish, his nickname is a direct reference to his heritage.
Ironically enough, his last name is Holland, a region divided in two provinces located in the Netherlands, another European country.
The surname Holland is of English/Scottish origin, not French (the French surname equivalent would be Hollande).
While it is stated that Irish met French on the boat over from Europe, French's accent is Cajun. John remarks on this after killing him and Welsh, telling Irish, "He didn't sound very French."
Given Leander's non-French surname, and his Cajun accent, it is possible that either Leander's backstory is a fabrication (whether perpetrated by him or Irish being unclear), or that Leander has lived in the United States (possibly, at some point, in Lemoyne) long enough to have lost his accent, and probably changed his name.
He is dressed similar to Irish, with the addition of a hat and a few other items.
Surprisingly, French can be disarmed, and even shot non-fatally. If disarmed, he will scream something along the lines of "I'm going to cut you like a pig, you son of a bitch!" but will not chase the player. He can, however, attempt to throw a punch at the player if they get close.
French seems to keep his revolver tucked at front of his pants rather than in the holster. The same goes for Irish.