Well, we’ve never seen Gali as a Matoran, Hahli was a Chronicler, and Nokama is now a Turaga. If you mean personality, There isn’t that big of a difference from what I could tell.
I feel that Gali and Hahli were a lot harsher than Nokama was. Gali started out a peace keeper, and while she did continue that she always did what was necessary. I mean, she did destroy the land of Karzahni.
Hahli was certainly different when compared to those two. She was adventurous, sharp, and above all, to the point. While she didn’t show it much in 2006, this really came out in 2007 when she basically took command of the team. How she handled the Barraki was much different than how most of the other mainline Toa of Water would.
On the surface, the three are very similar, all being remotely calm and wise in nature. I guess where the three diverge is that Hahli is a tad more shy and reserved, Gali is short tempered, and Nokama was more unifying and logical.
Gali was more of a soldier. She didn’t have as much of an aversion to violence as Nokama. She was perfectly willing to wipe out Karzahnni with a single blast, and it didn’t seem like she was too bothered about whether anyone was in it.
Nokama was a scholar, a thinker more than a fighter, much like the rest of the Metru.
Hahli was more impish than the others. She didn’t take things too seriously until the Pit.
In 2006, she never really stuck me as a warrior. She fought, sure, but she didn’t seem any more different than the previous two. While it’s been many years since I read the books, I do recall a scene where she called Dalu out for being too hot-blooded, saying that’s not how Ga-Matoran acted back in Metru-Nui. They were supposed to be teachers and act peacefully.
Then 2007 happened and she became “Hahli: Destroyer of Worlds.”
Hahli always struck me as a bit aggressive for a Ga-Matoran, from as early as she was introduced into the story (the Bohrok Animations) where she was one of the few Ga-Matoran (from my perspective) that was prepared to fight the Bohrok head-on. She always seemed (for me) to have a more defined personality than Gali and Nokama; Hahli might have even gotten more “screen time” than the other two.
Gali is a diplomat. She cares about others, and does her best to help them.
Nokama is a scholar, and for some reason she always struck me as slightly arrogant. As a Turaga, she was inpependent, not always going along with what the others did.
Hahli was an adventurer/rouge. She never seemed to really embody the pacifism and wisdom of the other Ga-matorans. She was a born fighter, and was very clever at finding ways to achieve her means.