We see him, in HGFinal, in Sento's universe. Which is the opposite of the one that all of the other Riders are from, as seen in the HG: Dr. Pacman movie. In Sento's universe, he Rider Kicks Enigma out of being cloaked, not in Rider form, not in Man At The Beginning form, but regular human form.
That alone shows a huge range of abilities.
Also, in HG: Dr. Pacman, he recovered Mr. Belt, as if it was no big deal. I don't want to spoil Drive, to explain why that's a big deal.
In Zi-O, he had the foresight to not only understand what was happening, but he was also able to send the Three Days In The Future Sougo with the Gaim Ridewatch, to Present Day Sougo, to help with the Another Gaim problem. He also teaches Sougo that you can't do everything for others, unless you want to stand alone. Kouta is shown to be fighting with others, letting them grow as people.
He is holding back his full power, for this reason. He could have easily dealt with any and all of the problems that I listed, but instead, he held back, and let the other Riders grow as people and as Riders.
You should stop being so willfully ignorant of what they are showing us, even in the limited amount of screen time Kouta has had, post-final episode. Kouta is rightfully called a Space God in the Drive/Gaim crossover movie, by Shinnosuke.
Kouta is way more than Adam from the Bible, as you claim. The Japanese don't use Christian mythology in the way that the West understands it. You also need to remember that Norse mythology is in Gaim also. You are supposed to understand the Golden Fruit as The Power Of The Gods, not as Adam gaining forbidden knowledge.