The Green Knight is superb character for inclusion in a Bretonnian army. He is reasonably costed and opens up your tactical options. The Green Knight is ideal for taking out isolated missile and gun powder units because he is ethereal and cannot be hurt by non-magical attacks (which missile and arty troops rarely have access to)
He is also very quick over the battle ground (being ethereal you don't have to worry much about obstacles !)
I would recommend his use in any game of 3000 pts or more. I would probably use him in games of 2000 pts but it would depend on which opponent I was facing and their armies most likely composition.
The Green Knight is cheap-ish, flexible and reliable - add him to your army.
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