I wish I actually knew who Hikawa was exactly instead of only knowing him as that badass Rider guy from Agito... :lol

Truth be told thats pretty much who he is as a character :lol

Really? :lol
Is he that character that's such a prick but is amazing and badass when it comes to fighting? :D

Nope he is a really really nice guy and is amazing and a bad ass when it comes to fighting :D

Hmm... I should watch Agito, shouldn't I? :D

Yes you should :lol

But when I have tried watching it, I can never finish the first episode. The part I always stop at is when some other guy (he was a very important character I believe) was swimming the pool and was drowning... and I can't remind anything after that... :lol

agito is reeeeally slow at the start. Once you see Shouichi be aware of the times he's fighting, which is very quickly, the pace picks up a bit. It gets a lot better once Gills appears too.

Perhaps the yokai can be part of the key powers to the ninja rider. But what key item for them to be used for?

I see the oni as the kaijin enemy. Perhaps oni generals placing masks on innocent people to become kaijin.

What about a bird rider since Jetman?

Bird rider? Well...OOO and Den-O have a bird forms, if that counts...
I think it's unlikely they'll make a theme around birds, though. Then again, I'm bored by birds, so I've never been big on them.