So, this guy Growlo or whatever, he was a leader of Holy Knights before Garling?

Considering that he is sharing the same position as Garling and Shamrock, it seems he is a big deal.
Note that he is no longer holding that position since whenever Garling took it.
Logically, the main reason why he would lose that position would be if he advanced as Garling did and became an Elder.
However, we know that Elders count only five...
So, he is either dead, killed by someone in the past or he is on a special assignment that made leadership over HKs inconvenient for him.
An assignment such as that of the man with the Burn Scar. This could very well be him otherwise I dont see a point of introducing someone like him here and now...