MY first thought was,
Gressiar wrote:but I think this character would be fun to play.
...And you're done.
That's a pretty solid reason to play a character.
But, OK, some further thoughts. First, I agree with Graymayne's basic point that, always in Multi-Disciplining, you have to think about how the two Disciplines work together - not just mechanically, but psychologically. But while I actually have a hard time seeing a Cavalryman that would also make a good Elementalist... an Elementalist who would also be a good Cavalryman (which seems to be the way you're going anyway) I can totally picture. (And I'm sure the other way can work, too, I'm just having a harder time with it.)
Especially since your plan doesn't put your character in the position of having to choose between their love of their mount and their love of.. the world, since you're really attaching a Cavalryman's devotion
to an elemental spirit, and that's kind of genius.
Since you're already an Elementalist, and I gather are planning to be able to summon your Ethrys from the get go, that kind of takes care of the, "Why would he ever have any other mounts?" question, too. (Although I have a totally different set of thoughts about that, especially the way 4th Edition built Cavalrymen... but that's a totally different topic so I won't get into it.)
The one thing that you might consider, though, is that a whole lot of key Cavalryman Talents actually won't apply to a spirit like that. Animal Training and Enhance Animal Companion leap to mind as things I can't imagine a GM letting you apply there. IF your group is using the, "Using All Talents To Advance" Optional Rule, you can just ignore most of those and focus on the Optional pick for each Circle -- you'll still have one or two irrelevant Talents to carry from First Circle, but there are enough good Options to be worth having...but there are good reasons why the Discipline Talents are the Discipline Talents. Those are some of the best things about being a Cavalryman. But as an Elementalist who only rides a Spirit, almost none of them up til 5th Circle are going to do you any good at all.
...Being that you are a
Human Elementalist, have you considered picking up a couple of Cavalryman Talents with Versatility, and just doing it that way? Charge and Trick Riding... depending on what you want out of the Cavalryman side, I'm not sure what else you'd need. They probably
do have the down side of needing to be upgraded more than you usually want for Versatility Talents, but it's still a lot cheaper than doing it through Skills, and certainly cheaper than carrying all the Cavalryman Talents you'll never use.
But you don't get the improved Durability that way, of course, and it might mess with the image in your head that makes it `fun to play'.