Here are some of my reasons;
1. Obviously he holds the Crest of Artorias.
I think he is holding onto the crest and selling it for a higher price of souls more to test undead and see if they can handle the task of making it to his grave and defeating sif. in it's item description it states " Many adventurers have left for the grave, but none have returned, for they make easy prey for local bandits. With such dangers, the crest can do more harm than good in the hands of the uninitiated."
2. He gives no insight to his past and only refers to himself as just an old smith.
3. Alvina tries to tell you the legend of Artorias is false although her and her covenant are the protectors of the forest graves.
4. Andrei says he knows little of Darkroot other than the divine blacksmith (possibly his mentor because he also uses divine embers) and that it houses the grave of Sir Artorias. He then continues to comment "Only, of those who ventured into the forest no one has returned...." Which furthers me to believe he has been watching over the entrance for some time and is now using the crest to test undead rather then let them charge into the slaughter.
5. Ingward, who is also out to test you by having you defeat a darkwraith (darkwraiths being the enemy's of Artorias) knows Artorias is still alive and tells you to seek him out and learn from him.
6. Going back to the crest, I believe it was created by Artorias' buddy Ingward because of the other part of the items description; "This crest opens a door in the Darkroot Garden sealed by ancient magic." with Ingward of course being the last great sealer.
7. It is said that Artorias entered the abyss and while there something transpired that caused Artorias to betray lord Gwyn and aid the four kings which would explain his sons fall from grace.
I definitely think the background and more in depth aspects of Andre were taken out to leave it as more of a mystery and will probably be shown to us in later DLC.