Putting a hero in you big knight unit with this virtue is the same as placing the army standard bearer with the unit for about 100 points less. Remember that this virtue lets you reroll ALL Leadership tests, including ones cause by Spells. The break test is also a Leadership test so you can reroll those as well. This could lead to situations when the unit around him is affected by a spell such as Blast and he was completely unharmed. If you take this virtue put the character with it at the front of your lance formations.
Hatred is a good thing.
If you decide to give your character this virtue then ignore the advice given above and don't give him the Grail Virtue. Hatred is just another form of psycology and therefore if he had the Grail virtue he would be unaffected. Every Knight unit should have a character in it that has this ability. Most of the Bretonnian armys hitting power comes from the charge so the ability to counter charge is an excellent one. Just remember that the fight is carried out in iniative order so get those Lords and Heros hacking.