The following comes from the THE HERALDS OF VALDEMAR WIKIA.

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Companions are highly intelligent magical creatures who look like (but are not) horses. They are easily recognized by their pure white coats, bright blue eyes, and the sound of their silver hooves, most commonly referred to as the sound of bells. Each Herald has such a Companion. Each Companion is white because of their connection to Node magic. They draw upon Nodes to enhance their abilities, giving them extended endurance, greater speed and the ability to Heal themselves quickly. For example, in cases of extreme urgency, Companions use the nodes to make short "jumps" in space --- an ability Yfandes describes as a "sort of Fetching" except with themselves and their rider--- which when coupled with their speed allows them to cover great distances in a small fraction of time. Some Companions can also enhance the abilities of their Chosen, feeding them energy from nodes and ley-lines. Many Companions are reincarnated Heralds.

Companions also seem to have a deep connection to the Star-Eyed Goddess, much like the Firecats have a connection to Vkandis.
 

COMPANION'S CHOICE

Companions "Choose" Heralds in the trainee or apprentice stage; the exact nature of the "Companion's Choice" is not stated explicitly in the novels. Heralds have described the moment in various ways: "being weighed and measured," "falling into [the Companion's] blue eyes," and "finding something I didn't know I was missing." The defining factor of one's Choosing seems to rely on the individual's moral fiber and where applicable, their ability to provide something that is needed by Valdemar, at the time. The Choice bonds Companions to their Heralds into a mutual relationship, and the bond can affect both parties' spiritual and physical well-being. The Companion becomes a loving friend who guides, supports, and sometimes augments a Herald's abilities. The joining is known to strengthen the Herald's Gifts, especially Mindspeech.

The Choice is, for the most part, permanent; it can be broken while in its early stages, although doing so will physically and emotionally scar both parties. After the initial phase, however, the bond becomes so important that the loss of either Herald or Companion is followed by depression and almost always death. Despite this a Companion can break the bond with their Chosen if at any time, they believe the Herald or Herald Trainee to have become unworthy. This is called repudiation. The most famous case of this was Tylendel, a Herald-Mage trainee and Herald-Mage Vanyel's first lifebonded. (Magic's Pawn)

Many Heralds can mindspeak with their Companions, but not all. The only time that a Herald might hear their Companion is the moment of choosing. Heralds Sherrill and Talia only heard their Companion's voice when they were Chosen. Herald Sherrill couldn't communicate with her Companion at all after that.
 

GROVE-BORN COMPANIONS

The Companions first appeared from a grove near the palace in Haven, seemingly in response to King Valdemar's prayer, though there are indications that the king, also a mage, may have cast one or more spells as well. Though the first Companions were all Grove-Born, later generations were born in the manner of normal horses. In "present-day" Valdemar (Reign of Queen Selenay), only three Companions are revealed to be Grove-Born: Gwena (Companion of Herald Elspeth), Rolan (Companion of Talamir and later Talia), and Rolan's predecessor Taver (Companion of Talamir).

Though the true nature of the Companions has not been fully revealed, it is inferred they are both the returned spirits of departed Heralds and Companions, as well as divine agents or avatars of the gods. In the novel Oathbreakers, the Shin'a'in goddess specifically refers to a Companion as the "child of my other Selves". That being said, Companions take great pains to reveal that they are both fallible and mortal. When a Companion or Herald dies, he or she is sent to a "resting place" where they can recover from their recent life. After a time they are given the choice of returning as a Companion, with all memory of their past life intact, or to lose all of their old memories and become a human baby destined to become a Herald. Not all Heralds are returned from previous lifetimes, many are "new souls" who are then Chosen by the Companions to join the ranks of the Heralds.

These spirits are not allowed to return as Companions within the lifetime of the people they knew to avoid emotional entanglements such as Choosing their own child, or revealing themselves to their former lover. When they return as Companions they often use some variation of their former human name (for instance, a Herald-Mage called Savil might use the name Sayvil as a Companion).

The Companion to the Monarch's Own Herald is always a Grove-Born Stallion, and is considered the leader of the Companions. This Companion, currently Rolan, has the ability to bond with a second Herald if the Monarch's Own dies; as Rolan has Chosen both Talamir and Talia (Arrows of the Queen). The Monarch's Own is also subject to this; if his or her Companion dies, it is possible for the Herald to bond to a newly-arrived Grove-Born Companion (Exile's Honor). Also, presumptive Monarch's Own who are also Heralds are only temporarily bonded to their first Companion; though the exact nature of this bonding is not explored (Magic's Promise). The Monarch's Own Companion is not a reborn Herald, They may be a true Avatar. There seem to be several who take turns. Rolan was Monarch's Own Companion during the Collegium Chronicles and Brightly Burning, two hundred years later Taver has replaced him, and Rolan returns when Taver is killed in battle.

The only Grove-Born mare since the time of King Valdemar is Gwena, who seems to have been "newly created" for the purpose of being the Companion to the first Herald-Mage in four hundred years. Being Grove-Born means she does not suffer some of the same limitations as other Companions. For example, not only can she feed Elspeth mage-energy, she can do so over any distance. There is no such thing as too far away from her Chosen.