I take "worst character ever" as a challenge. Anyone can take the best characters and win the game, I choose to pick the worst party possible and still win.
I've found that most bad characters fall into certain categories:
I think these categories are pretty good, though I would add a fifth -
* Poorly characterised in-setting. These characters may be OK mechanically, but are so irritating or uninteresting from a roleplaying perspective that taking them with you detracts from your enjoyment of the game. Or perhaps the plot itself ruins them, such as by unavoidably killing them off mid-game without warning, making all the XP and sometimes even the gear you gave them disappear, of you can't recruit them until too late in the game when your party is already settled. Examples: Sera (DA:I), Jan (BG2). Examples of plot-ruined characters: Yoshimo (BG2), Logain (DA

) Fawkes (FO3) or Aeris (FFVII).
As for the others, I've probably played Baldur's Gate 2 the most of any RPG I've owned, and I think there's at least one of each of these:
* Poorly optimised. These characters start out built wrong, and you have to either respec them (if allowed) or spend a long time leveling them correctly while they suck shit. Examples: Mission Vao, Merrill, Jerrick.
BG2 has several of these, but the worst is Jaheira. Fighter-Druid is a fucking useless combination in BG2. Druids suck in general in 2e anyway, but a F-D has the weapon and armour restrictions of a druid - no metal weapons (except scimitars for some weird reason known only to Gary Gygax), no metal armour, making her useless in melee. She can only use a sling at range, which is the weakest ranged weapon (doing 1d4 damage instead of 1d6 or more for everything else), and her spellcasting ability is unimpressive. I personally find her dour, charmless and generally annoying to be around anyway, but some people like her character.
* Complex abilities. Some characters have minigames associated with them or other mechanics that you have to spend a lot of time developing. Others have very situational abilities that are awesome but require outside research or hours of trial-and-error to find out. Examples: Gau, pretty much any blue mage in a Final Fantasy game, Cait Sith.
Edwin is annoying because of the hoops you have to go through to recruit him, which involves taking a certain path in the main game that (imo) is less fun than the alternatives. He kicks ass once you get him though. Any thief is at best situationally useful in BG2 due to the lack of stealth opportunities. You only need them for lockpicking and trap disarming. Jan, Nalia or even Imoen (if you get her the right gear) can cover those needs without needing a single-class thief, at least in the base game. That makes Yoshimo useless, his trap-setting ability is situational at best, and he takes up a party slot you could use for someone better. And of course he's also plot-ruined due to
dropping dead half way through.
* Overshadowed by someone else. Steiner is a straightforward character. He's a brick, but he's the best at being a brick. The problem is, he overshadows Freya and Amarant who also hit things for a living. Note that many of these characters do have things they do well, but most people dismiss them for their more famous colleagues. Examples: Many ff5 jobs, Eiko, Jean, Edge.
Minc and Valygar - holy shit rangers sucked in 2e. All Minsc's abilities are done better by other characters. Want to hit things with a sword for great justice? Take Keldorn. Want a bezerker? Take Korgan, his bezerking ability is much better, he'd tougher and he hits harder. Want a ranged fighter? Take Mazzy. Minc serves no purpose. Valygar sucks too, his kit is rubbish, and if you want a melee fighter to take down wizards, again Keldorn is better.
* Chrono Cross Syndrome. These character are legitimately bad. Edward before the remakes (FFDS edward is my favorite version), DK Cecil before the renakes, Harley, most Chrono Cross characters (hence the name).
I'm not sure what you mean by this as I've not played Chrono Cross. But Aerie is legit bad. She is mechanically useless and incredibly annoying.
If anyone wants to give BG2 a go (I would recommend it), I'd say take the following party if you want to powergame your way through:
PC - take a human Kensai and multi-class them to a mage at level 9. Take 5 pips in either axes or longswords and everything else in 2-weapon fighting. When you reach Athkatla during the tutorial dungeon, do Aerie's stupid circus quest to get the Ring of Human Influence for 18 charisma (meaning you can dump-stat charisma, replacing it with the Blade of Roses later on which does the same thing). Then dismiss your entire party, go to the scroll store at the top of the promenade, and learn every spell you can (save any scrolls you get up to this point). Your character will get all the XP from reading the scrolls to themselves and level up quickly. Once you hit Mage level 10, (which will not take long with 2e's weird XP system), you'll get your Kensai powers back. This will make you a potent mage with a much better than normal AC and the ability to slice someone in half in melee too. Seek out and learn Tenser's Transformation, which takes this build from OP to utterly broken, changing you from a powerful mage (mages are brutally overpowered in 2e anyway) to an unstoppable, basically unkillable melee death machine depending on what you need.
Keldorn - Keldorn is hilariously OP. His dispel magic ability is cast at twice his character level, meaning that by the end of the base game (let alone the expansion) he can cast it at the level of basically a deity, meaning that you never need worry about mages ever again. He can also wield Carsomyr, the best weapon in the game, which as well as being a +5 Holy Avenger, also casts dispel magic every time it hits someone.
Anomen - Anomen has a true powergamer's build. His fighter levels give him way more HP and DPS than any cleric, meaning he's a ridiculous tank and the only divine spellcaster you'll need. Take the "good" path in his personal quest and you'll raise his wisdom and, more importantly, shut him up. Give him the biggest shield you can find and The Flail of Ages (later Crom Faeyr if you can get hold of it). If you kit him out with the Amulet of Negative Plane Protection and the Mace of Disruption he'll mince his way through the hordes of vampires you fight in mid-game, solo. If you're playing Core Rules difficulty or above you'll need to find a way to raise his wisdom to cast 7th level spells - the Machine of Lum the Mad is probably the easiest way to do this.
Nalia - she's a wizard. You need wizards. But importantly, she's also a decent thief - good enough to get through the base game without any other thieves in the party in terms of lockpicking and trap detection.
Imoen - she's also a wizard. More wizards = moar dakka. She's a thief too, but not as good as Nalia. To use her as the party thief you'll need to get her the Ring of Danger Sense and the Ring of Lockpicks, and those aren't easy to get. So use Nalia, get her to sit back and nuke people with Cloudkills and Fireballs.
6th slot I usually reserve for characters you have to take with you for their quests (Cernd, Haer'dalis, Valygar, Korgan), and do those quests before I go to Spellhold. After that, take Mazzy or Jan.
Mazzy - People underestimate just how dangerous Mazzy is if you kit her out right. 5 pips in short bow is an odd choice, but there are some amazing items that synergise with it. Early on, give her the Tuigan bow (for +1 attack per round) and the Quiver of Plenty +1. This gives her a ridiculous number of attacks with infinite ammo. Later on, get her the Short Bow of Gesen and watch enemies drop like flies. Later when she gets her HLEs, give her improved whirlwind attack for 10 attacks per round. This turns her into an unstoppable machine gun of electric death.
Jan - Yet another wizard. You can't have too many wizards in 2e. He's also a better thief than Imoen or Nalia so if you're more into sneaking around he might be a good pick, either in place of Mazzy or one of the other mages.
Assuming you take Jan rather than Mazzy, this party effectively gives you:
4 mages
1 cleric
3 melee fighters
3 thieves
All with just 6 characters. Warning: this party is so broken that the game will have very little challenge.