I don't know very much about Vaclav, but I'll try to explain as much as I can. Vaclav was one of the first to join AX, so he's known Lady Caterina for a long time.

Vaclav is one of her best friends, even though Vaclav doesn't fight (as far as you can see) he's a very good asset to the AX team.

You don't see very much of Vaclav, either.