The first season was rough but decent, the second season is actually great, and Season 3... we'll get to Season 3.
To start off, much of the movie cast reprised their roles for the series with the exceptions of Wasabi (now voiced by Khary Payton AKA the voice of Teen Titans Cyborg) and Fred (now voiced by Brooks Wheelan replacing T. J. Miller.) Generally everyone was in character with how they were shown in the film with a couple of exceptions. Alistair Krei is the lesser of the exceptions, but felt more jerkish than he was in the film. He does get robbed kidnapped so frequently that he kinda felt like a punching bag. My bigger grievance among the cast is Fred. I feel that he made dumber in the series than he ACTUALLY was in the movie. It was kinda hard to overlook this especially how it relates to two other characters introduced in the series. Aside from Krei and Fred, I have no real complaints regarding the returning cast.
The new characters are a mixed bag.
Professor Granville served as a replacement for Professor Callaghan as he is spending time in lockdown for his actions in the film. Granville acts as a decent guiding hand for Hiro throughout the show though her role is more diminished in Season 2 onwards. Granville does have a personal connection to Season 1's main antagonist which explains why she invests so much time to Hiro.
Karmi is just fantastic. She fills in the role as Hiro's love interest, but they don't get along because they are too much alike each other. Despite not liking Hiro, Karmi loves Big Hero 6 especially Hiro's alter ego, Captain Cutie. She loves them so much, Kamri actually makes fanfiction (and later a web series) about them. Naturally Hiro feels uncomfortable about this, but the fanfiction does more good than harm as it gave out ideas and moves the team haven't thought to try with their tech. I was actually sad that she was forced to leave San Fransokyo due the event halfway into Season 2.
Mini-Max was made to keep Fred in check though that does not justify making Fred dumber. On the plus side, his superhero personality (in spite of being a robot) actually makes him a riot to watch. Despite being Fred's babysitter (even though Fred is not aware of this fact,) Mini-Max has proved himself to be quite an asset to Big Hero 6 to the point he was added to the opening animation for Season 2.
Megan Cruz was introduced in Season 2 and became the new love interest for Hiro after Karmi effectively left the series. I would be more upset this change if it weren't for the fact that Megan is actively trying to uncover the secret identities of Big Hero 6. This does cause friction between her and Hiro even though they do like each other. I wouldn't mind seeing more interactions between them, but then Season 3.
I 7@(%!#& hate Richardson Mole! He is only in the show to cause Fred (and Big Hero 6 by extension) pain and misery. If there is something Fred loves/likes, Mole will anything to ruin it for him. At one point he tricks Fred into revealing the secret identities of Big Hero 6 and forces them to let him into the team in fear of Mole leaking their secret to the public. The only reason Big Hero 6 got out of that mess was that Mole CONVENIENTLY got amnesia at the end of the episode. Should I mention that Mole has a crush on Go Go who's college aged and he's 11?
Obake is the main antagonist of the first season and arguably the whole series. This guy is why I initially compared BH6:tS to Teen Titins as he gave me Slade vibes. Obake uses other villians as pawns for his plans for San Fransokyo. He also has an unnerving interest in Hiro as the villian sees a lot of his younger self in the hero. Speaking heroes, Obake eventually learns of their secret identities and gives them out to other villains which comes into effect in Season two. Honestly Obake makes a great impression in Season 1 which makes how he goes out feel unsatisfying and out of character.
I gonna say it: Globby is the MVP of the series. Starting out as a wanna-be thief, Globby (real name Dibs) gets caught in an accident involving Honey Lemon's chem-purse and a neuro-transmitter and decides to become supervillain. He goes from a loser to a competent threat to Big Hero 6. Once he finds out Obake's plans for San Fransokyo, Globby pulls a heel-face turn by helping Big Hero 6 save the city. At that point, Globby becomes an actual superhero and stays that way for the rest of the series. He even gains his own nemesis in the form of Nega-Globby. Any episode featuring Globby always bring a smile to my face. Even the one in Season 3...
I'm mixed on Noodle Burger Boy. He is a recurring antagonist in the series thanks to Obake tampering with his programming. He is fine as an antagonist for the first season before out of focus in Season 2. My big complaint with Noodle Burger Boy is that he overstays his welcome which is especially true for Season 3.
While she didn't much when she was introduced in Season 1, Liv Amara really came into her own in Season 2 as arguably the show best villain. She makes deals with other villains by giving them new powers/enhancements in exchange for their service. Hiro quickly clued in that Liv was responsible for these "enhancements", but he could never pin it on her. She also the only villain to accomplished her actual goal (which I will not spoil,) but it's that success that leads to her defeat. I should mention that Liv Amara is the main antagonist for the first HALF of Season 2.
You might have notice I have avoided talking Season 3. That is because there is a MASSIVE drop in quality between Season 3 and the rest of the series. Not only is the episode count less than half the other individual seasons, but most episodes are actually mini-episodes which hurts the writing quite a bit. Many lingering plot threads were resolved in generally unsatisfying ways. Noodle Burger Boy becomes the main antagonist of Season 3, but I don't think he works as a big bad. If you want to watch Big Hero 6: the Series, I recommend sticking to the first two seasons as Season 2 does end on a high note.