I find Gattack insufferable. He's hotheaded but without the skills to back up. He perpetually gets himself and others into trouble seemingly without any ability to fix his mistakes or improve himself in any way. He whines constantly, butts heads with not only the other riders but his superiors and father and...well...he's just really annoying.
I see a lot of people say he was the real star of the show...but I just don't see it. Is it because he's an underdog? That people wanted to see him succeed over Tendou's incredible arrogance? Is it JUST because he was so different from the less-liked titular character? Because I don't think that's enough.
It's not a matter of liking him or not. Gattack was often the point of view character for developments in Kabuto's storyline. Revelations regarding the main story, from Hiyori's identity to even the origin of Kabuto's belt, were often given to the viewer according to what Kagami learned and saw, rather than Tendo. Pretty much the only running plot that actually followed Tendo was "Dark Kabuto kidnaps Hiyori".
Now, in the end, that's kind of murky, especially due to Inoue's writing, who did treat Tendo as a more standard protagonist, unlike Yonemura, resulting in things like Tsurugi's inconsistent character arc (I'm Tendo's rival! No, I'm Kagamin's best friend! No, Tendo, you're the only one who can kill me!). Yonemura's epilogue chapter for Kabuto is Kagami focused too though.
Anyway, I don't see how Kagami didn't improve. He was beaten up a lot, but, unlike other secondary Riders, that was just building up to his evolution. He eventually comes to be relied upon by Kabuto, holds on and later defeats Nagi, after initially getting beaten up without even transforming, and he's the one who disarms the final enemy and carries Tendo for the Double Rider Kick. He wasn't just beaten up to be kept away from the main story, it was part of his character arc. Now, it's true that he was kind of an idiot, but yet again that was part of his character arc - learning to stand up by himself rather than following others, something he only completely gets in the final episode.