You can at any time invent up to two allies for your character. An ally is someone willing to do the character a favour or otherwise help him out. It could be a family member, an old friend, or someone in debt to the character.
An ally can also be an organisation or other cohesive group of people, such as a trade guild or a criminal gang.
The cost of bringing an ally into play is that the GM gets to invent an enemy for the character. A rival or someone the character crossed in the past. Someone out to harm the character.
The enemy you gain is the GMs choice, though you can make a suggestion. The enemy gained will be roughly equivalent to your ally in regards to power, though not necessarily in the same way - your ally may be a good warrior while your enemy is a good schemer. So if you decide that the king of the realm is your ally, be ready for having the ruler of a neighbouring realm as your enemy - or the brother of your king who is scheming to overthrow him and already has half the realm behind him.
Both allies and enemies are meant to remain in play and appear again. They will stay both in the fiction and on your character sheet until dead or otherwise permanently removed from the characters life.
If an enemy is killed or put out of commission while the ally remains, the GM invents a new enemy.
If an ally of your character is killed or put out of commission, you can choose to replace him with a new ally, or leave the slot empty. While the ally slot is unfilled, the respective enemy remains inactive.
These enemies are personal enemies of your character. They are in addition to whatever antagonists are part of the story or whatever enemies the party may have made.