The maximum of companions available for fight is twelve. You will learn about eight of them later in the game, whereas the remaining four become available through diplomacy. Below, you can read descriptions of six of them, who are the best in my opinion.
The main character that will always have to take part in a journey (although not necessarily in a battle). The young sovereign is highly versatile and useful. His first skill Command grants 20% maximum armor. Kay can use this skill every turn and, when it is cast on a powerful tank, effects are excellent. Apart from that, his attacks are strong and he can support others by casting banner that increases damage dealt. Kay is excellent in any party and he helps survive the toughest of battles.
The magical Aliss is one of the most useful characters in the game, Her Firefly deals 60% basic damage and skips from enemy to another. She can cast a flame barrier that deals damage to enemies that cross it. She deals lots of damage also using the Inferno AoE spell. However, her greatest asset is the Lord of the Flames, which helps in any battle. Aliss is an excellent character for any party.
A soul imprisoned in a powerful suit of armor. Levant is a walking fortress. In the game, there are several tanks, but in my opinion, Levant is the best of them. He is incredibly resistant to any damage, he can form phantom walls, knock back enemies and teleport around the battlefield, using his Holy Driver. Also, it is a good idea to cluster your characters around Levant, because his Holy Nova provides the party members with lots of armor. Finally, his ultimate skill, Cry of the Brave, makes all enemies in the battlefield unable to use their skills for 1 turn. Levant is perfect for the first line of attack and unarguably, it is a good idea to take him to the party.
The savage and fearless Signy is one of the best dps's in the game. She has AoE skills: Pounce is highly useful as a dash towards the enemy, and Whirlwind deals immense damage to enemies within range. She has an immensely strong skill, Masscre, which lands four strong attacks on selected enemies. If there is only one enemy within range the four attacks are landed on him. This is a perfect skill for eliminating, stronger opponents and useful in the battle.
Grenn is a typical rogue that remains in shadows and deals deadly damage from there. His Crippling Strike imposes Poisoned onto enemies, whereas Diversion imposes Slowed Down and blinded. Grenn can also stun enemies and avoid taking damage. This is a perfect companion for the battle, where you have to incapacitate a single, strong enemy. Also, he is useful when you have to relocate your party into specific fields and eliminate a specific enemy.
Diego is a self-assured archer. However, this goes hand-in-hand with his mastery - he makes a highly valuable addition to the party. Diego collects charges of "Overconfidence" using Quick Shot. Overconfidence can be used to buff up his Tar Shot and Cobra Shot. Thanks to this, he will deal more damage and slow down enemies more effectively. His ultimate skill, Money Shot affects multiple enemies and has a considerable range. Diego is especially useful when you have to defend against a larger group of enemies.