These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Complete Monster: Captain Grumpf appears to be little more than a comic relief villain at the start of the game. Later on, in Basset, he attacks an orphanage to coerce Red into giving him the Crystal Stones. Once the stones are given to him, he decides to hell with it and continues to open fire on the orphanage and everyone there, including a fellow Kurvaz officer, all For the Evulz.
Bruno as well. He is more than willing to brutally torture and murder as many people as it takes to take control Lares. Its worth noting that, of all the villains in the Shared Universe of Solatorobo and Tail Concerto, Grumpf and Bruno are the only ones who actually die.
Fridge Horror: Elh seems relieved when she loses her immortality, because it means she'll age along with everyone else and won't end up in a Mayfly-December Romance with Red. However, Red is a Hybrid, and Baion, the original Hybrid who provided DNA for all the others, is well over 350 years old. Maybe she should've kept it after all!
Though it's never stated whether Baion's "children" inherited his immortality or not, so it's possible the horror isn't quite as bad as you might have thought.
Game Breaker: The Dahak Mk II and Trance aren't much of one by themselves, but once Red learns to "Trance" at will, he gains the ability to launched ranged attacks when in Trance mode. This makes fighting Bruno and other bosses absolutely trivial in New Game+ runs. The Type-R'sSpiral Floater technique can also be this against the first half's bosses, as their powerful ground-sweeping attacks can't touch you at all while using it.
Surprise Difficulty: One downloadable quest has Red help out with teaching orphans fire drills. It starts with a really cliché safety quiz ("What do you do when a stranger asks you to come with him", etc.) and a small obstacle course with fire hazards. It ends with a much more difficult obstacle course with fire everywhere, falling debris and crabs trying to kill you.
Fridge Brilliance: Since the only reason Red had to run that course is because he was running his mouth and bragging about how easy the first one was, it's logical that the course was nigh-impossible, just to put him in his place.
Viewer Gender Confusion: Elh◊ looks like a girl (despite that Skirt over Slacks look being claimed as boys' clothing by Red), but gets referred to as a boy by the characters in the game, and in the Japanese script uses male pronouns. Later, it's revealed she's a girl after all.