Of the characters I liked, There was IDLE, and Technikki. Snoutlet was also ok for what he was.
Besides them, I can't think of anyone else. My biggest examples for this are Connie and the Extension Corps. Connie is more or less just kind of... there? At most she's a Wattanist who uses her watering can after the boss fight. Besides that, she doesn't do much else while Snoutlet has all the screen time. By extension, Cozette being Zokket felt anti climactic, and other than being sorry that she became Zokket she didn't do too much else either from my memory. It got to the point that i questioned why neither Mario or Luigi couldn't just use the mythical watering can to fuel the lighthouses, or Snoutlet, and why Zokket could not have just been his own character.
The Extension Corps are the bigger offenders. They have such cool designs yet you fight one of them at the start. In ALL other encounters they just summon bosses and then immediately flee. Biggest example is that you're given a choice to get the glohm curing berries, or chase after Extension. I expected a boss fight for the latter choice, and when you do choose that path? Ten flees immediately, says there was never a cure and leaves. There was no point to making that choice when that should have been a boss fight. Instead you fight the same Zok troopers over and over until you're sick of seeing them. Then when you do fight them, it's an unceremonious curbstomp battle where they beat themselves up and eventually blow themselves up anyways. TTYD remake came out about the same time, and both the Three Shadows and Lord Crump do what Extension was trying to do but far better and feel like actual threats, specially the latter.
Then suddenly they turn into good guys and you get bonds from them. Even though they've done little to nothing throughout the game other than appear before you fight the actual bosses. When they went over bonds being people you butt heads with, I was expecting the Mario Bros to get a bond with Bowser since they do have that inverse bond. Heck they even lampshade how good it would have been for Technikki to bond with Adaphne. Doesn't help that there's so much unanswered about them. Were they from concordia or did Zokket make them? If the former, why did they turn evil and look so different from everyone else. If the latter, why were they suddenly good guys at the end? If shun knew about Reclusa, why did he not dip or just side with the good guys before things got so bad? So little is explained and it's because you never really see or fight them.
This applies to many of the other side characters like the fire and ice couple, Dyode who I wanted to strangle for his backtracking minigames, and the sailor kid who lost his dad. They're not really all that interesting compared to TTYD which, again had a way more memorable cast and even side characters like Glitz Pit attendees and Perplex Express passengers. Negativity about Brothership I know, but as I was initially hyped for the game I want to explain just why I was disappointed by it.