Puzzle Quest: Challenge of the Warlords

Companions are gained from completing certain side quests theses quests will let you know if they're for a companion by having a name in parenthesis. There's a total of 9 companions throughout the game, 5 are needed to complete the achievement requirements. Some can be missed through the campaign as well as there are 2 added through the DLC add-on taking the total up to 11 possible companions.
Red quests mean main quests and Green mean side quests so do all those once you run out do a main quest then try again.

On a side note companions can leave if you somehow anger them or intrigue them to leave. This can happen near the end where you can choose to go down the evil path or good, if you go down the evil path they'll keep leaving you one by one.

From SCA KAZUMA @ Xbox360Achievements.com

• Syrus Darkhunter - Just play through the game. He's the first person that joins you during an early part of the main storyline and can't be missed. After you complete the delivery of the Sorcerer to Maethelburg, he will join you as you continue north.
• Princess Serephine - Instead of delivering her to the Trarg for the King of Enmouth, take her to Bartonia and she will join you.
• Khalkus - Again, playing through the game normally will unlock this character for your companion list. He is the one you must help to protect the caravan. After protecting the caravan, a brief dialogue will ensue. Afterwards he will join you.
• Flicker - When you unlock the ability to fight against Mechataur, there will be a series of side quests. They take place around three areas surrounding Dragonrealm. Complete these, then go back to Dragonrealm, and take the quest to complete them again. After this, return to Dragonrealm and fight against the Minotaur attacking Flicker. After a brief cutscene, Flicker joins your party.
• Sunspear - After you defeat Mechataur, you come to the Realms of War, where you meet Tauron, a Firewalker (a breed of minotaurs). He sends you to two places, and after you return from these two places, you meet Sunspear. He guides you in your next quest to piece together Lord Sartek. After the brief conversation, he joins you immediately.
• Patch - This character is unlocked after doing a series of events after you meet Lord Bane for the first time. These events are located in Hyann. After doing three relatively short missions for him, he joins your party.
• Winter - You unlock this Winter Wolf after a few quests revolving around 2 areas, Ylarie and Ehlariel. You come across these cities while closing in on Lord Bane. After completing these few tasks, Winter asks to join you, saying you have helped his pack and he wishes to return the favor.
• Sunspear's Spirit - After completing a series of side events revolving around Sunspear's choosing of a final foe, he leaves your party and tells you to return in a week. Walk around a bit on the map until the green glow emits from Ragnar's Coast, and he will join back with you, with a newer power. Where he once gave you +10 starting red mana against minotaurs, he now gives you +10 starting red mana against minotaurs AND the undead!
• Elistara - This High Elf lass joins you after you end your search for her dragon mount, Crystal, and you report to Kelthurax, the Dragon Leader. After the short exchange of words between Elistara and Kelthurax, she joins your party without even a cutscene leading up to it.
• Drong - This gigantic ogre joins you after a long series of battles during a quest to bring him, you guessed it, food. Each creature he sends you to slay you must bring back to him so he can eat them. He even asks you to bring him a fellow ogre to eat! This fat, sweet gigantor joins you after you bring him a sea serpent to eat. Gotta love the lovable fat guy!
• DLC Galnoth - This Dark Elf joins your party after your first attempt at going after Araveine. Your character will force him to join you for reasons I won't spoil here.