Colress was Team Plasma's "leader" after N left. He definitely had a reason for being there. But what did he do? Let's see...
He's Chaotic Neutral. He wants to learn how to bring out the full potential of Pokémon. Seems harmless enough... But he's willing to do anything it takes for that goal. Doing experiments and messing with the Pokémon? Yeah he's probably done it. But defeat him enough times and he just goes, "Oh, your Pokémon are that strong just by you being nice to them? I'll try it!"
He's such a great character to include because in gen 5, everything's a world of binaries. Cheren and Bianca are black and white. Hilda/Hilbert and N are opposites; the chosen ones destined to clash. And then there's Colress, a colorless, neutral entity in a world of black and white. B2W2 focused a lot on this idea of "neutrality" (Marlon saying "This has nothing to do with me, so no thanks!" as an example) and Colress is the prime model of it.
He definitely didn't need to be a part of Team Plasma for the sake of his own research (in XY an NPC mentions they saw him traveling around the large stones so he definitely does his own thing at certain points). But Ghetsis is a clever man. Ghetsis spent years and years traveling, finding what and who he needed in order to reach his own ends. Meeting a person like Colress, who also would do anything for his goals, was likely great in Ghetsis's eyes because he could offer the Team Plasma research facility to him and be able to make use of his research. In the Plasma Frigate Colress even says, "I was asked by an acquaintance to help with his research."
If anything, maybe detailing Colress and Ghetsis's encounter, and just exactly how Ghetsis convinced Colress to join Team Plasma, would add to Colress's character. By seeing their interaction, Colress's desire for knowledge could become even more apparent based on how/what methods Ghetsis used to get Colress to join him. Colress claims Ghetsis is an "acquaintance," but just how acquainted are they? Their decision to work together could even give us some more solid backstory on Ghetsis, too.
To conclude, Colress is one of my most favorite characters of the entire franchise, in terms of appearance and character, because he really exemplifies just how neutral "neutral" can be. He's cool, he's ruthless (when he needs to be), he's intelligent, and he sits right in the middle in a world of opposites.
