Reminds me of OOO, Eiji wasn't necessarily dull (in comparison, he had more backstory than Kouta does at this point), but every other character was so lively and colorful, it really made him seem boring most of the time. Especially when compared to Shotaro and Gentaro who surrounded him, who were pretty much the most fun characters of their respective shows probably.
In his defense, Kouta's had some funny moments and I love the body language of Gaim, so he's not at the level of Wataru/Haruto for me. I'd say he's like Kenzaki, a completely substandard lead without any real notable traits, but has a decent enough attitude and character that works (and hopefully has THAT one big moment that cements him as a bro).