"Hellboy (Ron Perlman), his pyrokinetic girlfriend, Liz (Selma Blair), and aquatic empath, Abe Sapien (Doug Jones), face their biggest battle when an underworld prince (Luke Goss) plans to reclaim Earth for his magical kindred. Tired of living in the shadow of humans, Prince Nuada tries to awaken an ancient army of killing machines to clear the way for fantasy creatures to roam free. Only Hellboy can stop the dark prince and prevent humanity's annihilation."
Hellboy was pretty much focused on the character development of its titular character, Hellboy II: The Golden Army now uses that development to actually move on and focus on other characters. It's one of the main reasons why this sequel works, and why Guardian of the Galaxy Vol. 2 was so lacklustre; all of the character development from the previous film has been retained, allowing the story and lore to grow and evolve, unlike the second Guardians film that decided to undo all of its character development and start from scratch (last time you got to see them overcome their differences and learn to work together and become family, now you can watch them...do it again!). Allowing development of the supporting cast helps bring more depth to Abe Sapien and Liz's characters and makes the film more of an ensemble cast (multiple principal characters with roughly equal time spent on them)
With better equality in the film and more focus on developing the characters equally, it allows more time to look at the work they do with the BPRD (the Bureau of Paranormal Research and Defense), and we actually get to check out some more missions with them investigating paranormal situations, building up the supernatural world in this universe.
An aspect that has changed quite drastically is the public perception of Hellboy. In the first film, there was a sense of wonder and intrigue around the possibility of Hellboy being real, as such, when put face to face with the red being the public were more amazed than anything else. In this sequel, Hellboy's existence has been confirmed, and the public perception is much more negative, mainly consisting of fear and anger, which wildly contradicts what we have previously seen. This new reaction from the public is necessary for a section of the plot but was still unexpected.
The negativity is necessary because the big conundrum for the heroes is deciding what side of a war that they will side with. Do they side with the monsters that are being wiped out and having their habitat unknowingly destroyed by the human race, or do they side with the humans that have raised them, and they have spent their entire lives trying to fit in with? Much like Hellboy's previous decision on whether to be the evil he was born as or the good that he taught and raised to be, Hellboy and his team are now in a situation where the side he is on is actually in the wrong, but the opposing side is planning a genocide to protect themselves. It brings forth a very difficult decision to make, that I wish the team were given more time to languish over.
As far as villains go, Prince Nuada was quite well done. Despite telegraphing his weakness right from the start, his character was given a strong motivation that the audience can certainly empathise with. but is it just me or do elven-like characters generally make really poor villains? Perhaps I'm just annoyed at the poor performance of Dark Elf Malekith from Thor: The Dark World, but it always feels like so much effort goes into the make-up, that little thought is given to emotive performances. Nuada ended up being very much one-dimensional, and while the fight choreography was exquisite as per usual, I would have loved to have been given more backstory to understand why he was willing to wipe out the human race when the rest of his family were ready to die.
Surprisingly, despite the extra character development that they receive, Liz is one of the more unlikeable characters of the film, at least until halfway through the film. Mainly due to another source of conflict needlessly added to the film (not dissimilar to the previous love triangle), which is introduced purely to create conflict; though I love that Hellboy has maintained consistency in this area. Abe, on the other hand, gets his own story arc, but while his emotions do feel genuine the events that he goes through to reach that emotional state do not feel adequate, making him come across like a desperate love-sick puppy.
Hellboy II: The Golden Army overall is very much in the same vein as the first film. With a great mix of practical effects & CGI and a better balance of antagonists, the audience gets to appreciate a wider scope of the Hellboy universe, with the introduction of many new creatures, and new hidden worlds.