Alvina is the leader of the Forest Hunter covenant.
Alvina distrusts Shiva of the East, fearing that he might use her covenant to accomplish his own goals. This stems from removed content from the game in which Shiva attempts to kill the player.
Alvina is a large cat resembling a Great Feline and is capable of speech. She seems to know Knight Artorias and his compatriot Sif personally, and leads the Forest Hunter covenant to guard his grave. In the past, she guides the player to the location of Sif that has been surrounded by Humanity Phantoms behind the illusory wall in Chasm of the Abyss.
She will give the player a Divine Blessing after killing one trespasser; holy water from Goddess Gwynevere, daughter of the great Lord of Sunlight Gwyn which indicates that Alvina was in contact with Gwyn’s pantheon at on point.
During the events of Dark Souls, Alvina resides in Darkroot Garden, perched on the windowsill of an old ramshackle building. The path leading to the Darkroot Garden goes through the old abandoned church near the Undead Parish, but Darkroot is considered a forbidden plane now. Darkroot Garden attracts all sorts of “lunatics” according to Andre of Astora. It is rumoured to house the grave of Sir Artorias the Abysswalker and this rumor is in fact true. Darkroot Garden is the ruins of Oolacile, a human town that was corrupted by Manus and the Abyss. Long ago Artorias attempted stopping Manus and the spread of the Abyss.
Artorias, according to Elizabeth, had nary a murmur of Dark. He was overcome and deeply scarred by the Abyss. In his last desperation, he used his Cleansing Greatshield to form a barrier to protect his compatriot Sif. With his arm broken and his will all but subdued, Artorias fled to the township of Oolacile. Alvina, likely unable to help Artorias in his state, guided the Chosen Undead to Sif’s 'prison' in the Abyss.
After Oolacile fell, Alvina’s dedication to Artorias and Sif endured. While Sif inherited Artorias' Greatsword and took up his post as guardian of his grave. From the Sif's reaction to the player if he has rescued Sif in the DLC before triggering Sif as a boss in Darkroot Garden, it can be assumed that Sif is protecting the grave to prevent others from entering the Abyss.
Shiva of the East explains the rules of the Forest Hunters;
Fight and hunt as you like. Whoever's fastest gets the prey. That's the way we do it. Only… Don't forget what Alvina said. Traitors aren't given a second chance, for any reason.
As hinted at in Shiva’s dialogue, the Forest Hunters of this day and age are perhaps a little overzealous in their duties. The entire Darkroot Garden has become the territory of the Forest Hunters and ironically, these warriors are considered to be local bandits. The Forest Hunters are very dangerous to the uninitiated (those not in the Forest Hunters covenant), Shiva tells of how the first person to make the kill gets to strip the corpse of valuables. It would seem that Alvina cares little for how the clan protects the forest graves, as long as the job gets done and that even those innocent of attempted robbery are not exempt from their belligerence.