Bulletproof
He was bulletproof in every sense of the word. Obviously, in the sense that bullets couldn't kill him, but there was more to it that that.
Upon finding out about his former identity Seras started to think about how bulletproof her Master really was, had to be. To be turned from Master to Slave in a day. It must take such resilience, to be that bulletproof, in an emotional sense.
Seras had always prided herself on being an emotionally strong girl. She had remained in Hellsing for 15 years. Being shot at and shooting for 15 years, 15,600 bullets, and 7 guns. It took a strong mind to stay at a place like Hellsing for 15 years. But after 15 years of being a weapon, Seras had to leave that place.
But not him. He was just that, a weapon, a killing machine, a personal bullet holder and a slave. But he stayed so strong, so bulletproof.
When Seras was younger she had admired his physical strength, the fact he was literally bulletproof. But as she got older she began to realize what really made her Master strong was his mental strength, his ability to remain bulletproof when every race in the world had shunned and disowned him made Seras truly respect him.
So while Seras sat there on the cold wet ground looking above and ahead at her former Master taking 15 rounds of contact explosive silver rounds for her Seras finally realized exactly why she still called Alucard 'Master' after all of these years.
She thanked him, loved him and owed a lot to him, yes.
But above all she respected her bulletproof Master.