I thought Agito was only loosely connected to Kuuga.

I think there's only 1 or 2 comments in the first few episodes that people say shows they are connected. It it VERY LOOSELY connected.

Hence the "somewhat."

They specifically mention it's been 2 years since No. 4 (Kuuga) defeated the Unidentified Lifeforms (Grongi), that a UL hasn't appeared since, that G3 was made to combat the ULs if they ever return, and they compare Agito's powers to No. 4's powers. It's intended to be in the same continuty.

… Isn't every series of Kamen Rider connected? You know, in the same way every series (more or less) of Power Rangers is connected ?

All Showa Riders are connected directly. Kuuga and Agito are connected by dialogue. Ryuki-Kiva are stand alones. Here's where it gets fun. Decade has a multiverse of other versions of all Riders where none of them are connected. The Sen-O world is THE Den-O, however. Meanwhile, Black and Black RX are 2 different characters even though it's the same guy (and even the original actor!).

Confused yet? Yeah? Good, there's more. W started a new continuity with Foundation X intended to link the shows as a new enemy for all Riders (like Shocker in the Showa era). This continuity, if you consider Let's Go Kamen Rider to happen on this timeline, uses everything as canon. Even Decade and his multiverse of confusing crap.

Btw, much like Transformers continuity, this is all debatable because there's no offical continuity. The only things we can say for sure is that all Showa Riders are connected to each other and that everything from W onward is connected to each other. And some people would even debate that.

PS Super Sentai is only one continuity as every series mets the last series in crossovers, though that didn't become the norm until the 90's. Plus, Gokaiger united them anyways.

Black and Black RX are the same character, I thought...

By that logic, Decade connected every series as well.
At the end of the series, when the main Riders come telling Tsukasa he f*cked up? Those were the originals, not the AU Riders.

Not in Decade and they both appear in Let's Go, so it's only the same guy in the original Showa continuity. Of course Let's Go already had about a dozen paradoxes in effect, so we could probably say he's fighting alongside himself.

How can you tell the difference when thay never demorph?

Well yeah, we can tell it's supposed to be the originals.

The original Blade transformed to fight, and he was an ally to the original Kiva. The only one who charged at him in the finale that wasn't the original rider was Yuusuke Onodera, who was the Kuuga in Decade.

Plus the whole- Apollo Geist destroying all the other AR worlds, and Decade defeating him destroyed Kiva and Hibiki's AR worlds.

Okay, got it. A wizard did it. :p