Well, here's my team (with explanations for all of the characters), in the order of my line-up
1. Darkling. He is the most mobile character with his 9 speed, ability to fly, and both shadowports, and he move anywhere on the map. This allows him to get behind characters and destroy opponents with backstab shadow blast/mind blast.
2. Shadowboxer. He is a pure offensive monster. Because the opponents are often grouped together at the start of battle, he can destroy them with shadow vortex. Probably the best offensive non-main character.
3. Festus. Due to my extremely powerful team, a healer isn't absolutely necessary, but Festus is nice in some of the final battles to keep everyone alive and fighting. I've also kept his attack up, so he can do some pretty decent damage.
4. Main. It's not like I have a choice
. My main has maxed out PsP and Health points. The only opponents he can't take out with a backstab Pyro Blast are Tastidian and Nelis (and possibly Cerzak, I haven't tested that out yet). The best character available to you by far, with amazing staying power, extremely powerful attacks, and very good speed (I always use the green orb).
5. Dorgon. I have a glitched Dorgon because of a save game transfer. He has one attack, called Mind Blast, which is actually the laser beam attack. He also has 7 speed. This combination means he rarely gets touched, and he can always hit somebody.
6. Gamblin' Jack. I have struggled over this spot on my team. I chose Jack because 1) He gives me the freedom to side with whomever I want, and 2) With his recover ability, he can stay alive basically forever. Jack's Snake Eyes attack also can deal extreme amounts of damage.
7. Niven. The slowest of the offensive shadowlings. Niven has one advantage that ensures his place on my team: Trick Shot. He isn't hurt by Tastidian's or Nelis's attacks, and, unlike Shadow Vortex or Pyro Hail, Trick Shot can actually deal backstab damage, even if it doesn't always work.
8. Grotius. With 7 speed, Grotius gets outmatched by Shadowboxer and Darkling. However, his powerful Frost Blast attacks make him an excellent teammate. This way, I can line up Gamblin' Jack (Snake Eyes), Shadowboxer (Shadow Blast), and Grotius (Frost Blast 2) all behind an enemy and deal massive damage.
And now, for those who are not on my team:
Anya. Well, as I've already written, a healer isn't absolutely necessary on my team, so when it comes down to it, Anya loses out. She just isn't as mobile as Festus, and I need a healer that can always reach my teammates.
Guy. Guy is the toughest one in this section. In the past, I've traded him in for Gamblin' Jack. I no longer use Guy because 1) With low mobility and no healing options, he often gets hammered and dies, and 2) His attacks really aren't anything special. Jack simply has more staying power, plus the ability to heal others.
Flint. I've also used Flint before, and I really like him. He's a powerful tank. My only issue is with his speed. As you may have noticed, I built a quick team. (Jack is the only one with speed 5, if the main has the green orb). Flint just gets left behind, or worse, he slows others down.
Helena. I've never seen Helena as a viable alternative to Jack. I think she is just too awkward to use, and even if you can use her properly, she's not that effective. For offensive teammates, I think everyone else outclasses her.
So, there you go.