Ouja was a pretty good villain, but I've long thought (and sometimes heard) that he was originally intended to be killed off sooner and only kept around because Bandai really wanted to try the push for a fourth main Rider, an idea they skirted on near the end of Agito and finally went all the way with in Blade. But it was an after-the-fact decision rather than something decided from the beginning, and it kind of shows in the designs. From the get-go, Ryuki, Knight & Zolda were all intended to main guys, and thus were designed to look unique from the others. They get the colored suits with black arm/leg armor, rather than the black suits with the arm/leg armor being the colorful part (look at a picture of all 13 lined up and you'll see what I mean.)
Then there's the fact that each one's helmet intentionally has a design element of the iconic Rider image (Ryuki- the eyes, Knight- the crusher/mouthpiece, Zolda- the antennae.) The others generally have helmet designs recalling a villain (Scissors- Kanibubbler, Ouja- Cobra-Otoko) or referencing something else entirely. There was a lot of thought process put into making the big 3 particularly distinct from the other 10.
But putting aside the design aspect, it just feels like they didn't know what to do with Ouja after a while. In his early episodes he's scary as hell. He's the first real legit threat since they blew their chance on Scissors (still, IMHO, Ryuki's best villain on paper next to Verde), and Gai's basically got a gimmick that'll last about 4 episodes ("let's play a gaaaaaaame!") Early on Ouja's taking hostages, playing dirty, and Hagino has that smug grin dialed up to 11. He's a scream in his debut. Then Knight Survive kicks his butt, and we get a couple weird "maybe-he's-not-all-that-bad" story lines that do nothing more than reaffirm that yes, he is all that bad, but he can turn it on and off to mess with others.
Then Tiger joins the show, and there's where I really got the feeling that Kobayashi was trying to do the psychotic anti-hero thing again, but actually treat it seriously this time. Tiger feels genuinely disturbed. He really does believe he's a hero despite doing monstrous things. Compared with that, Ouja's basically a sneaky bastard who keeps getting away with everything because everybody around him lets him get away with it. He's one of the nuttier Riders, but that feels to me like more of a conscious decision. He's bad, he's aware he's bad, and he's bad because he loves being bad. In other words, he can help himself. It feels to me like Tiger can't.
I'm okay with the bad-for-the-sake-of-being-bad thing. Ouja's meant to be a hero in the cause of evil in that way Apollo Geist or Shadow Moon are. But it didn't seem like Kobayashi or Inoue were really all that interested in him once this becomes evident, but they were under mandate to keep him around anyway. A lot of what he does could be given to other characters with basically the same result. Tiger killing Imperer (which he basically does) would have even strengthened the scene, since Imperer really did come to trust him just as the others had.
Keeping Ouja around does give Zolda somebody to fight later on, but were it up to me that's where I'd have used Verde in the series, because I've always seen him as the dark version of what Kitaoka could turn into- a man letting his wealth and power go to his head, unchecked by friends or a moral center like Goro. Inoue would eat that up, since he loves that trope.
I mean just look at how much Shinji & Ren care about Asakura during the last few episodes. Um... yeah. Then there's how he comically gets tied to just about everyone in the Ryukiverse. It was convenient enough having him be the guy who injured Tezuka's friend after establishing him as having a history with Kitaoka, but by the end you can practically play six degrees of separation. He has in the past pissed off or will in the series piss off just about everyone in some way. That guy gets around! Or Kanzaki sure knows how to pick 'em. I dunno, but I think he'd had been more effective if he'd been more of an unknown or only kept to being on a few characters' to-kill lists.
In fairness, Ouja's always kept watchable by Hagino and Okamoto, but it always seemed to me the original idea was for him to be the big bad for a chunk of the series, then get defeated so other Riders could take center stage (basically, what they did with Weather in Double.) The short version: I like him a lot, but I wouldn't have minded if he'd bowed out about 15 episodes earlier.