Long, red locks, muscular, lean and tall plus a somewhat snarky attitude, what is there not to like? At first he was merely a bad guy but then soon his story is told. Once I had learned more about Renji the more I came to like him.
He is stubborn and loyal almost to a fault. His childhood in soul society was hard and full with both good and bad times.
His charming ways or rather lack there of are really amusing. The way he gets picked on but still takes it good naturedly . He has a strong sense of justice and will do what he thinks right. He is hard working and really loyal to his friends and squad.
I really like how devoted he has been to Rukia over all the years that they have been together. I can completely relate with the way he works hard to improve, the way he struggles on even though it does not come naturally to him. He has worked hard for everything he has, from his position to the respect of those around him.
The way he is not afraid to think out of the box and will willingly take his opponents head on even if he knows that they are stronger than he is and that he has a high chance of getting his ass handed to him, is impressive. I also admire the way when need be, he will swallow his pride and ask for help when he needs it.
Then I am absolutely hooked on his long hair and tattoos. Plus the fan art is not too bad either.