I don't know much about the behind the scenes stuff with these movies, but I justassume from how little Inoue cared about actually putting Kamen Riding into The Next, he really didn't care about continuity with The First. You are right, though, Asuka's absence is incredibly glaring if you watched The First.

Where can I watch kamen rider drive episode 0?

You can download that over at Over-Time subs.

It's very weird. I read on the Kamen Rider Wikia that Takeshi split with her before The Next. That pretty much makes his rivalry with Hayato pointless in The First. To see an important character just disregarded like that is puzzling.

When do you think chase will become kamen rider chase?

whats the episode of Agito where the guy who is psychic healing his and everyone's wounds and is sweaty all the time dies?

that was the last episode I was on.

According to the Kamen rider wiki this would be episode 31 so you would be on episode 32 if my sources are correct.

Was Kamen Rider Gaim have a happy ending or bittersweet ending

For me, it was bittersweet because not only Mitchy turn good, he also reconcile with Kouta and his brother but he end up losing Kouta and Mai when both of them become Adam and Eve. He lost 2 people that he was closer to them

It wasn't a happy ending for the main cast, after everything that went down. Definitely bittersweet, as 2 characters are still alive (Just kind of...Gone), another suffered brain damage, and Michy is...Yeah...
The side characters definitely drew the long straw when it came to endings.

No ending with a dead Yoko is a happy ending. :(