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.
YMMV: Tear Ring Saga
Game Breaker: Shigen's personal sword, Dullahan, revives him with full HP whenever he gets killed. This is not a one-time thing, in fact, it can actually happen multiple times in the same turn, and the only downside is him losing his attacks in that combat. Basically, as long as the sword lasts he's immortal, and in a game with Final Death that's a really big deal. You can affortd to be a lot more reckless with him than you would with anyone else, hell, he functions as a pretty effective meatshield in a pinch. Just be careful not to use the sword too much early on.
Narron have Elite as his starting skill, and have mounts. Which is basically everything he need to be an overpowered character.
Goddamned Bats: The Opus/Ahriman (depanding on which translation you're going by) monsters take this Up to Eleven. They're weak, and have reasonably low HP, but decent Speed and a 'weapon' with 0 weight, meaning they're pretty hard to double and they double you surprisingly often. They can fly, so weak characters in the rear aren't safe, and they're probably going to get doubled too. But what's the worst thing about them? Theymultiply. Leave even a few alive and they'll become a massive swarm in the space of a few turns. Their King Mook form can even spawn more. (up to 4 per turn, which can still divide the turn they're spawned!) Did I mention you have to go through an entire chapter filled with them?
There's also the Thief Sword toting Myrmidons in Map 26b. Anyone they so much as scratch loses one of their weapons. Forever. They can't steal *-ranked weapons, but equipping one woon't stop them from cleaning out everything else in your inventory.
Guide Dang It: Recruiting Leteena involves doing several things over the first half of the game in specific ways that are never mentioned anywhere in the game. And once you do manage to get her in your party, she'll leave again anyway and take one of your characters with you unless you have a certain non-mandatory character in your party!
The Scrappy: Maerhen since he has low stats, and his burglar skill lowers the stats of nearby allies.
Now that the semi-complete translation is out, it's safe to say this might be the case In-Universe too. The guy gets the Butt Monkey and Accidental Misnaming treatment even before you recruit him, and afterwards it's implied the 'lowering stats of nearby allies' thing is because they can't stand being around him!