He's quite dead. But dead men usually are. He's also an angel. Sort of.
Unlike the Biblical angels, Crüxworld's sort-of angels seem to have been mortal men and women once. St. Croix, "a mongrel of French and Spanish blood", certainly was. He himself doesn't even comprehend this, but most of the time he doesn't even remember who he is. Nevertheless, he seems to be aware on a basic level that he is a creature of stagnant death, and powerful at that too. Though somewhat simple on the outside, he has a cunning and intelligence that work frighteningly well when he is provoked by some sort of catalyst, be it death, life or someone talking to him. St. Croix is known to repeat things as if they were the truth and the answer, and this is especially true when he doesn't comprehend why someone asks him what he means by something.
St. Croix is devoted. To what, he doesn't know - but whatever it is, he always responds to anything that could threaten the thing he is devoted to.
That being, the dead. Being dead himself, he is bound to them, and being dead and bound to something by some force, he is also subject to some rules. One of them is the rule of summoning. Summon him formally, and he shows up. This doesn't necessarily mean doom or death to the one doing the summoning, nor does it mean he will obey.