Honestly, I usually just take characters from the same series and wait for the "sparkle" to occur. I'm not really even sure what effects the koma bestow or which ones are best of even just good. 
 
I realize that although my tactic may work well enough, I'm limiting myself. For example, say I wanted to give Ichigo from Bleach more aerial maneuverability, and I'd like to add either a triple jump, an air-dash, or both. How would I do that? Or maybe I want to give Goku an immunity to poison? Maybe give Jaguar higher defense against swords?
 
I feel like there needs to be a definitive guide as only the GIant Bomb community can put together...

If you go to Genroh's faq on Gamefaqs and then ctrl-f "Help Character" it'll eventually bring you to the section where it tells you what all the helps do.

 It's been a really long time since I played that game so I don't really remember anything specific about Help Komas. Still, rather than looking to put together a completely new guide on Giant Bomb, if there isn't one already, why don't you just google one? 
 
Actually, I just did it for you and came up with this:  http://shadowlink1134-fmj.deviantart.com/art/JUS-Help-Koma-Effects-Chart-59210387 
 
Someone on deviantart made a pic that have all the help Komas arranged with the pictures from the games as well as their effects. It's pretty big. I'd recommend saving it and zooming in on the file, worked fine for me.

Oh, I've been searching for good ones all day, but they only give general overviews of the systems and compatibility. I went by that one, but when I click on the link it didn't work.
 
Update: I updated my java and it finally appeared.
 
Edit: Also the guides that are already out there are kind of scattered. It would be beneficial to have a more cohesive guide. But of course, everyone's going to argue that it isn't needed because "no one plays it anymore, there's guides out there, you're an idiot, you're ugly," etc.