SOME IDEAS ON DIMITRI BEING A SPECTRAL AND HOW THAT RELATES TO THE SITUATION
Dimitri probably left the club by choice because of any of these:
1. Huge injury from a battle
2. Possibly lost ability to see spirits by something natural of physical
3. Just doesn't want to be involved
Most people think 1 is the most logical answer but there is no sign of injury on Dimitri, if Dimitri were to get hurt from a spirit, even if it was a year ago as most assume, it would have been fatal, seeing as spirits can be very powerful. If the spirit was THAT powerful Dimitri would have had to tell his parents about spirits and spectrals because he was hurt by one, maybe even Doctor Zarei couldn't fix the injury because it could have been internal, and not external. Which makes a difference when it comes to spirits. Which leads to 2, maybe the injury was so physically bad that Dimitri lost the ability to see spirits and spirit energy and all that because of the hurt. And as it was subtly referencing with Isabel and Dimitri interacting somewhat, something happened. who knows what happened honestly but the attack is the best bet I'm done ranting